OK i've found were the issue lies, because your back bet is being placed at the best price and getting matched straight away its not counted as a fill/kill order.
There's 2 things you can do
A) On the parameters tab increase the fill/kill time and place the back bet at 'custom ticks above the best reverse market price' 0
That will offer the bet at the reverse price, if matched the offset will trigger - another bet won't trigger until there has been 30secs since the back was matched and there are no unmatched bets
B) Continue to place the back bet at the best market price but instead of using a fill/kill time condition, you'd need to use signal
ie, set a signal with the back bet and have a signal changed condition to test its been 30 seconds since the signal was set