Time Machine won't recognize Mac after hiatus

I've cruised around this discussion section and spent a lot of time on the Pondini site (thanks Pondini!) but this problem seems not to be covered.

Everytime I go out of town for a couple of weeks (taking the MBP 13" Retina)—when I come back the MBP won't link to the Time Machine backups. It insists on doing the whole thing from the beginning. I have regular backups going back to February, but "enter time machine" shows files only since I last returned.

This defeats the whole purpose of Time Machine!

So, (a) why is this happening? (b) How can I reassociate with older files so that TM sees the whole range of my backup files (c) How can I make this dis-associatation stop!?

Thanks in advance for your advice and assistance.