If you doe looks big, maybe she's too fat. Fat does have trouble conceiving and giving birth, it's just not healthy for them.
Second. don't leave the buck with the doe, it's not safe for either, the buck will pester the doe too much and the doe, being bigger could attack your buck. Don't be lazy about how you manage your rabbits and breeding them.
When you're ready to breed again. Put the doe in with the buck in the morning and watch to make sure they have a successful mating. Pull her back out until later that evening, put her back in with the male and repeat the process, then remove her. In about 28 days she should kindle. Put a nest box in for her a week before her due date so she can begin nesting.
Best bit of advise I can give is to LEAVE HER ALONE while she's kindling. Rabbits are pray animals... if they are disrupted too much while giving birth you could cause her to delay birthing, which could cause her problems. I don't check my doe for 24 hr after I think she's kindled. And I only look long enough to see if they're alive, and pull any that may not be. Rabbits are prolific because they have a high mortality rate. losing babies or having a rabbit that is not a good mother is not unusual. I give my rabbits 3 tries at mother hood and then I wont use her anymore if she can't get the hang of it.