I dont have a motorised boot, but have had similar reluctance to lock. In my case, there were two different causes:
Failed spring on the bottom catch- this resulted in the striker being ever so slightly low and no soft close function, replaced the spring
Sticky latch mechanism- there is a microswitch inside the latch which was gummed up, so a quick dismantle, re-lube and adjustment to the microswitch resolved it.
Judging by your description that you heard a motor noise, my guess is on the spring. You can use the old one in a pinch by bending out one of the coils or get this
http://www.maplin.co.uk/p/150-part-s...sortment-n06ct and have more springs than you know what to do with!