Doing NROLFW and almost done with stage 1. I'm having trouble with the prone jack knife.

The last 4 workout A in NROLFW stage 1 I have not been able to progress in the prone jack knife. At this point I should be able to do 3 sets of 12, but I get so tired and my form starts going to shit after 8 so I'll stop. Could this be because I'm steadily upping the weight I use in the other exercises?