I've seen this many times.
Some scientists, in particular senior scientists, can be outrageously busy. They might have to be on your co-author list because they are responsible for the data. They might insist on being in the loop for the data analysis, but when it's time for feedback on exactly that, they have no time. They may be completely drowning in emails. They might rarely pick up the phone because they're constantly travelling or in meetings. They certainly don't have time to read a paper draft related to some data they provided to someone junior months or even years ago; they may not even remember. Do not take it personally just because they're not communicable. I've found that some scientists are eager to take on a lot more work than they are able to do (because everything is awesome!), but underestimate the time it takes. See Hanlon's razor and Hofstadter's law. Scientists are human, the expectation from a senior scientist may not be human, the result is what we observe. I've had situations where I thought a senior scientist had lost interest, but then spoke to him for 5 minutes at a conference and soon found out that he was still quite interested instead — just incapable of allocating time.
Some senior scientists are very good at time management, manage to reply to emails and even give detailed manuscript feedback even when they're no less busy than any other senior scientist. In my experience, they are a minority.
How to prevent this? If possible, seek out less senior co-authors. That will reduce the risk of this happening. I've had better responses from co-authors junior enough to desperately need papers to have a chance at graduating/staying in science/etc.
How to get out of this situation? Try to reach them in other ways: phone their office, phone or email a secretary to find out when the scientist may be in, or if you're lucky enough to both attend a conference any time soon, try to seek them out during a conference break. Prepare some clear questions: do they still want to be on the paper? Are they happy for publication to go ahead? In my experience, the result is usually one of either:
- Please go ahead and remove me as a co-author, I don't have time. I'm OK with the data being only acknowledged/cited, instead of full co-authorship.
- Please go ahead, keeping me as a co-author. If other co-author X approves the draft, you can assume I'm fine with the draft as well.
- Let me introduce my postdoc/PhD student who is happy to take over my role on this study (we will both be co-authors).
- I will have time in X days/weeks/months, please wait for me then.
I've seen all of the above, sometimes with an added apology. I would be very cautious of the last answer, because in my experience, it usually ends up being not true.
If you reach them and they say they don't have time, you could even suggest if they might have a PhD student or postdoc who they could introduce you to (see point 3). Personally, I think #3 is the most professional answer.
You may need to press them, or ask a senior colleague to press them, to get one of those answers at all.