First things to do is set up work benches to ensure you have a full coverage to be sure.

Then you have to look at the ditch/moat itself. If the edges aren't hard they mobs will be able to get into the ditch. Then if there is a anything they can walk up (and sometimes it is surprising what they can) they will go right up. So if you don't have outer walls they'll come right in. This is why I start by ensuring I have full coverage and a little over do it and then I slowly remove benches and replace them with other items that prevent spawning. However you also really have to make sure the benches cover the ditch moat too to make sure they don't spawn in it.