Losing parked calls

I have set up my system and have managed to remove all of the kinks from it, apart from one issue.
Using asterisk v1.4.18-3, combined with the HUDlite programme, when a call is placed into the parking lot it is fine, however if you try and place another call in the parking lot, from any phone, the first call is lost altogether.
Was wondering if anyone had a solution to the problem or could point me in the direction of what is causing it to happen?? as ive been trying to sort this for a couple of weeks now.