Parking lot wont allow more than one call

Ive set up my parking lot with 8 possible parking channels (71-79) and the time out is set to 10 mins.
However whenever I try and park more than one call in the parking lot at any one time the first call will be lost altogether. Ive looked through all the .conf files and the only thing I can find is that the information for the parked calls and the transfer of calls is in two different files, I dont know whether this is relevant or not??
Does anyone have any clue as to why this may be??