OPK thoughts

Ladies! I have a question!! I have been looking some stuff up online so see what could be the problem. I noticed in the morning I have a dark positive opk, then when I retest later in the afternoon it's very faint. I have read that FMU can give you a false negative. My opk says to take at the same time everyday but not when. So, what I'm thinking longer cycle (33-41) less of chance of a positive right now and I shouldn't use my FMU... Thoughts? Experiences? Help?