place bets in running not being taken up help please

[facefish1]

I place a simple bet in running where if the bet price is 50% of initial price a win bet is suppose to be placed .
ive tried a few things best market price 2nd and 3rd best market price
1 point below best market price
2 points below best market price but just cant get the bet accepted
any suggestions on best way do this
some bets do get placed but these seem to be mainly because I havnt killed the bet and if it isnt filled it seems to pick it up if the horse drifts back out.

i was gonna play about with fill and kill bets but thought id ask here first

just reading through forums seems like one of my problems maybe because im only placing 50p and it takes betfair 24 secs to place a bet under the allowable limits is that correct ??
i believe also that using pratice mode the underlimit bets go on but when live dont always but i would have thought 50p bets wouldnt be a problem
not sure how to use offset bets would that be a better way to do this ??
cheers