Tactics

One of my main chess learning tools is doing the tactics trainer. I try to do my limit of 29 puzzles everyday. What I do not understand is how I continue to not improve my score. I have a low score in the 260s and a high one of 640s. However I am down about 150 over the last month and am staying in high 300s-low 400s. I average consistent 40% right and 60% wrong. I am not sure how i could be losing tactical knowledge. Maybe I am learning that i do not know what I think i know?

I must admit I do the session fast. Like a blitz game and often do not go through all possibilities.