Why can grandma talk sometimes and other times not?

Could be a bunch of stuff. Hearing first of all. And when you remove other signals (like vision) or add other problems (even tiredness), as well as background noise or other people in the room, hearing ability can change.
But so can cognitive ability. People with dementia/Alz have good days and bad days. Days when they seem just like their old self — like the veil has been lifted — and other days when everything is a struggle.
I suspect that whatever is going on with your grandma is coming first in her verbal ability. She may know, on some level, that she doesn't know what to say, or she may not be able to make the words. I had a serious head injury years ago and was unable to speak for a few days, and I remember knowing what I wanted to say but there was a disconnect between my brain (the words) and my mouth (saying them). So I just didn't talk because it was exhausting.
It's okay if she's not talking. If she's communicating with you and seems fine, I wouldn't push her on talking unless a doctor said she should.
Old age is hard. Help her communicate with you in any way she feels comfortable.

With dementia at play, communication gets VERY hard; both with her speaking AND with her understanding language (aphasia). It gets hard for your grandma to follow a conversation, even when she may appear to BE following along, she likely is not able to. As the disease progresses, she'll be able to use fewer and fewer words until she's down to using one or two words entirely. TV is too much stimuli for her to process, books and reading material is the same; as her comprehension of words decreases, everything sounds like mumbo-jumbo in her mind.

She will begin to sleep more & more as the disease takes over her body & mind. That's the nature of ALZ and when you combine it with Parkinson's, it's a double whammy that's taken hold of your dear grandmother.

She'll have good days & bad days on her way to the final stages of the disease process, so enjoy the lucid moments as you can.

I suggest you read this 33 page booklet (which is a free download) which has THE best information ever about managing dementia and what to expect with an elder who's been diagnosed with it.


Understanding the Dementia Experience, by Jennifer Ghent-Fuller 
https://www.smashwords.com/books/view/210580



Here is a list of useful tips for you to use when dealing with grandma nowadays:

The “Dont's”
· Do not reason and argue
· Do not demand that they reason or problem-solve
· Do not demand that they remember
· Do not demand that they get their facts straight
· Do not correct their ideas or scold them
· Do not reorient them
· Do not think that they are being uncooperative on purpose
· Do not think that they really do remember, but are pretending not to
· Do not use a “bossy” dictatorial attitude in care
· Do not act with impatience

The "Do's"
· Enter into their frame of reality, or their 'world'
· Be aware of their mood or state of mind
· Use few words and simple phrases
· OR use no words, just friendly gestures and simple motions
· Do everything slowly
· Approach from the front
· Wait for a slow response
· Constantly reassure them that everything is 'OK'
· Keep people with dementia comfortable 'in the moment' - every moment
· Maximize use of remaining abilities
· Limit TV or radio programs which they may feel are frighteningly real
· Maintain privacy
· Provide a safe physical environment

Language Needs
· Use short words
· Use clear and simple sentences
· Speak slowly and calmly
· Questions should ask for a “yes” or “no” answer
· Talk about one thing at a time
· Talk about concrete things; not abstract ideas
· Use common phrases
· Always say what you are doing
· If they repeat their question, repeat your answer as you did the first time · Give them a longer time to process information
· Wait patiently for a response
· Be accepting of inappropriate answers and nonsense words
· Speak softly, soothingly and gently

Care Needs
· Recognize that receiving personal care feels intrusive
· Reassure with your tone and manner
· Do one thing at a time
· Talk through the care “play-by- play”
· Be aware of your body language and use it to communicate relaxation and reassurance
· Be sincere
· Use a soft, soothing touch
· Be aware of the individual’s unique triggers
· Be aware that a person with dementia may not accurately judge whether a situation is threatening to them
· They may respond to fear, pain or anxiety by defending themselves with what we call “aggression”
· If they become distressed, stop immediately and allow them time to calm down – don’t try to restart the activity right away
You need to change your behaviour to adapt to the dementia because the person with the disease cannot.

A good book to read is Living in the Labyrinth: A Personal Journey Through the Maze of Alzheimer's, by Diana Friel McGowin. Learn all you can about AD/dementia b/c knowledge is power!

Wishing you the best of luck with all that's going on.

If she is very old, she might be approaching the time to leave this world.

After taking care of various dementia patients in my family I concluded that sometimes they just don’t want to talk. So leave them alone as they are slipping away and don’t owe anyone a conversation. It’s too much for them.