I get this sometimes and I think the reason behind it is different for everyone. Like for years I've focused on lucid dreaming on and off and for a long time (it still happens on and off) I get stuck in a spell where I'm becoming lucid often but my control is quite limited and I feel like I'm just going through the motions of realising I'm dreaming, flying around, trying to make things happen, failing, almost constantly stabilizing. Really it feels like I'm drugged out of my mind, just flailing around unable to do anything interesting. When I am stable, I'm disinterested because I know if I try anything I'll wake up. Thankfully things are better lately. Anyway, I think since most of my dreams had been "blah" for awhile it was all I came to expect even though in waking life I would get myself all pumped about the idea of LDing. This was enough to achieve it but not enough to beleive in my abilities while dreaming. Just keep with it and you should get through the dry spell. Also, if your RL is dull and you just feel numb in general (like you're semi-depressed) I find this leads to crap dreams, probably because of low self-esteem and just a lack of emotions in general. So yeah, there are lots of reasons. I'm sure you have your own.