I was having that problem with my Marshall 425a cab (The Vintage Modern cab). My singer/co-guitarist had the same problem with her 1960 cab.
I can tell you what the problem was with hers and mine and I'm pretty sure it's the same problem as with yours...
Marshall outfits their cabs with
cheap plastic caster sockets which wear out after a short time (mine was only after 2 years of gigging/practicing 1x/wk with a NEW cab)
My solution- I found a seller on eBay that makes a set of metal ones which he makes specifically for Marshall cabs so you don't have to drill new holes. He guarantees them for 10 years. They were a perfect fit.
(here
http://www.ebay.com/itm/Marshall-4X...ultDomain_0&hash=item1e6b69923f#ht_614wt_1184
Yes, kinda expensive for what it is, but it really does work. I think he started selling them at $89 a while back, but realized no one would buy them at that price.
If you're not concerned with drilling new holes/, you can just get a set of industrial casters at the hardware store.
Hope that helps!