Perhaps the one you're forgetting is the one you need the most. And there may be a subconscious wish not to think about that. I don't know; you'll have to look at it yourself and see if you continually leave this particular one out. Sometimes, if we are having some difficulty with our concentration and mindfulness, we may go through the first ones and always leave off the last one. That's understandable. It may be helpful to recognize that and make a resolution to ourselves to make sure we do a very short form and then perhaps go to the last one first sometimes.
I noticed I happened to do this during one retreat. I was getting very involved in all of the daily reflections except the last one. I realized that this could possibly put me off-balance, especially since it was reflecting on "to care or not to care," which I needed a lot. So I made a resolution to reflect more about this. That might be what's going on. It depends on which one you're leaving out. Or it may be that we forget because the mindfulness has been lost at that particular moment. The fact that you do it all the time seems strange, though, so you may want to put a little bit more effort into rememorizing.