I'd suggest double checking that you're not just going through the motions on RC, mantras etc Really feel the RCs, make sure the mantras have the gravitas to get into your dreaming mind.

When we're learning something new and exciting, it can be easy and lucid dreams can just flow. But after that we need to put in that little extra effort

As people have suggested, you can try different techniques, just to shake things up a little bit if nothing else if you feel you're in a rut.