Simplest answer to your questions of why people aren't scheduling follow up calls later is because it takes time to implement new ideas, and by the time they have implemented them, they've either

A) forgotten about their call with you. They've probably talked with many other people since then, helping to further refine the idea, etc.

B) they may have now moved into a phase that they don't think you can help with (i.e. they have new problems now, and there's no reason to waste money just to update you on the resolution of a past problem)

C) they're just shy

In all cases, the best solution is to just reach out to your previous callers maybe a month or two after your call. Send them a Clarity message and ask how the ideas you talked about worked, and where they are now, and whether they have any new problems that you might be able to help out with.

best of luck,

Lee