For anyone that stumbles on this thread, the male also seems to need feeding again if it manages to get free.
It's possibly something else, but I was feeding 3 females, 1 got 10 portions and was instantly pregernt. The other 2 are ready to mate, have now eaten 10 portions, but just never actually gregnant. As soon as I added a trough to the pen with the male, the females were all instantly pregernent.