This is the problem, and the solution!
Dust is getting into your phone and settling on the contacts for the charger. It's been a problem for many sony ericsson phones, but all you have to do is scrape the dirt off, it settles in the exact spot where the charger contacts sit, so you don't get contact. If it's not charging scrape the contacts on the phone with a needle, lightly then blow, then conect and boom your away.
I'm so sick of trawling through these forums looking for answers, why do people keep posting the same problem over and over and over, and no proper solution? So anyway rant over, I hope this helps everybody.