I've observed that some people give an answer to a question requesting some information from the original poster but sometimes forget to check that question. Are they even notified when I comment to their answer? If not, is there a mechanism to "ping" someone with the link to the original question?
Yes, they are notified when you comment on their answer (assuming they're on the site at some later point).
I usually try to get back to questions, but often the OP doesn't respond for hours or days and, honestly, I've moved on. The question may have additional answers by that time and it may just be not worth my time to follow up on what I had requested. People are notified about comments you leave on their answers, but not with respect to comments to their comments on your question. This is particularly insidious as often, when I leave a request for more info I do it as a comment since I don't have enough information to answer. Again, I will usually try to check up on these, but if nothing is forthcoming in an hour or so, it's probably too late. It's really up to the OP to make sure they keep up with their question and get information out as quickly as they can when more is requested.