Maybe the unconscious part of your mind was busy with other things at the time; it does have rather a lot to look after.

I have certainly also had failed experiences (actually, probably more failures than outright successes), and although I'm sure many of them were due to lack of expertice (I haven't had all that many lucids yet), some of them may have been due to other things (like the unconscious being busy with regulation of body temperature, or digestion, or gradually awakening the body, or ...or ... or). Maybe the best advice is simply to expect, that whatever you undertake will actually work ...