Hi
Just a few more thoughts:
The trigger logs will show if and why the error happens. Can you confirm your initial back bet in the Place market is always there?
The trigger needs to look up a matched back bet in the opposite market, so effectively your lay trigger will be delayed by at least one refresh of the list of bets, by which time the price may be gone.
You refresh rates look ok, try updating the bets individually for each market to see if that is an improvement.
If any of your bets are below Betfair minimum, the time taken to submit the bet will be at least 3 times longer than a single bet.
Perhaps try to offer a lay price higher than the triggering price to increase the chance of getting matched:
in the price field use "best" according to the settings Betting Options > When laying at best price...
or in the price field use r_ticks(lay_price,x) where x is number of ticks higher than lay_price
Betfair will always match the bets at the best available price.