This is my first topic (and hope not to be the last

I've used betfair before, but for the first time i'm using betangel software (the trial verison for now) and despite reading the user's guide there's something that i don't understand.
When I place a bet with greening and trailing stop, the stop moves with the odds but the stake does not.
As far as i know, (imagine i back and then lay) if you want to green the trade your stake must be calculated by the back/lay ratio, so if the lay decreases, the stake should increase, but betangel calculates the stake just with the back and lay odds from when you placed the first bet.
Anyone knows what to do??
Thanks a lot!