DS reliable in pants but not trousers! Help!

[Failedspinster]

I'm currently potty training DS1, aged 2 and 9 months, in the house. i started off letting him run round pantless, then as soon as he was reliable put him in pants, but no trousers. he's now reliable in pants, so this week I've started putting him in pants and trousers, and it's just been accident after accident. Normally he can tell you when he wants to go without prompting, but if he's wearing trousers, not only does he not say, he will say when asked that he doesn't need to go...then minutes later he will have an accident!
I'm completely mystified as to why he is utterly reliable in pants but then has accidents as soon as trousers are added. Has anyone experienced anything similar, or got any ideas what it might be? Help!

What are the trousers like? Does he have to fiddle with buttons and zips?

Have you tried him with some light elastic acted waist type trousers that are quick and easy to pull down?

The trousers are elasticated at the waist and just need pulling down - he still needs (and asks for) help to manage his pants, so I would expect him to do the same with his trousers - but he doesn't :(

Have you tried just trousers with no pants? My da associated having something on under his trousers with having a nappy on and so would just go. Take away the pants and that feeling of support has gone. Worth a try?