I find that if you ask someone when they can catch up, there is always something on and that you can't catch up till two weeks later. BUT if you just drop in on that person, or send them a quick text "hey, in the area, can I drop by for a coffee?" they are available.

There must be something about us thinking about all the things we have to do when we have to think ahead or maybe it is yet another commitment, that is just more than we can't cope with right now.

Personally I find I catch up with more people being impromptu, rather than "booking it in". Don't worry, I don't just drop in on those that I know hate it

If you look at some of the older generations they do tend to drop in to catch up on friends and family without calling first (well that is what I have observed anyway)