Do you tell and convince yourself that you will remember your dream the next time you wake up? I also like to do a “day recall” when I go to bed (replaying in my head what I did today… its a good exercice for memory, quit similar to doing Dream Recall !!) . I was doing those two things for the last 2 months, and stopped for about a week, and my dream recall has been a little lower for the last three days. Might be related.
About vitamin B6, I know it will highly affect your brain activity. But (in my case), it sometimes “annihilate” my dream recall for that night (sometimes, maybe 3-4 out of 10) BUT, when I recall dream after takin B6, those dreams are really vivid and the DS are coming in large bunches (so many of them!!).
About B6, check that topic below, lots of stuff there:
Link to Big vitamin B6 Topic
Have fun!!