Hi
At the best of times prices move very quickly in-play, and with the very low liquidity at the moment the prices will have massive gaps and be all over the place.
Betfair will match at best available price, so the way to increase your chance of getting matched is to offer a back bet at lower than the current best price.
Try using a back trigger with the price set to r_ticks(back_price, -x) where -x is your acceptable number of ticks below current back price. You can also use "best" without the quotes in the price field. This effectively puts in a back bet at 1.01 and will be matched at the best available price.
OR
have a look at the trigger action: match unmatched back bets
OR
look at the Stop Loss options in the settings
Spread loss zero's the loss on the traded selection with a much higher liability on the other selections.
