Can't straighten legs in straddle ups

Hi.  I just graduated from C1 Tuck-Ups to C2 Straddle Ups, in Foundation Core.  I feel like I'm doing them correctly, but with one HUGE exception -- I can't keep my knees from bending at the top of the movement.  Instead of being straight, they are closer to 45 degrees.  Assuming that the knees cannot be bent, then there is no way I can progress, despite the fact that I easily accomplish the rest of the movement with good form.  What do you suggest? 

@Chip Srull How is your hamstring flexibility? Are you currently doing the Front Split and Middle Split courses as well?

I am able to perform Jefferson Curls with and my knuckles touch the floor (knees not bent).  I have started doing the middle splits course once per week, but it seem like it will take forever for me to get past 90 degrees!  Should I start doing middle splits 2X per week?  What about my Straddle Ups progression -- they are easy if bend my knees; but seems like I will never progress if I must keep knees straight.

10 hours ago, Chip Srull said:

I am able to perform Jefferson Curls with and my knuckles touch the floor (knees not bent).  I have started doing the middle splits course once per week, but it seem like it will take forever for me to get past 90 degrees!  Should I start doing middle splits 2X per week?  What about my Straddle Ups progression -- they are easy if bend my knees; but seems like I will never progress if I must keep knees straight.

Hey Chip, looking to your "post-history" appears that there is no post about form check...so my suggestion is ALWAYS HAVING a form check first. if the problem is the range of motion we need to check it. so if you can post the IM1 and IM2 is better..i mean windmill and jefferson curl.