There could also be physical factors. If you have a full bottom and feel a definite need, and still can't do it comfortably, make sure the nappy is not too tight. It sounds silly but apart from the difficulty of actually pushing the poo out into the nappy, it can add to the sensation of not being the right place to do it that MakABDL relates.
Or, if you are trying to use the nappy at a convenient time to enjoy it, when you don't really 100% need to do a poo, you might be expecting results that are simply not ready to happen. I don't know how much freedom you have to do this, but if you just keep yourself diapered, eventually you will have a full-on need to do a poo that is difficult to resist. As soon as the urge strikes, use it to your advantage; squat down and have a nice long steady push and see what happens.