This happens quite often, and isn't anything to worry about. In its most extreme form the mother will finish trying and start walking off (with or without a baby) before the father has even arrived at the bed/couch. The peeps will often move at different speeds, partly depending on their energy levels and on their likes/dislikes.

Provided the couple share their kiss that starts them off to the bed/couch, a baby will or won't appear depending on luck, not their timing.

If your peeps are having trouble having babies you should have them checked by the doctor to see if one or both might need Baby Boost.