I think it might be the wetness. You might try wiping the dipstick when it gets drenched a few times with a paper towel and see what happens.
This just hit me; In the missionary position, lean back so that you’re sitting on your legs, don’t use your arms to support her legs. Do something like rub her clit, but she may involuntarily kegel while tensing up so she can keep her legs up.
Also, try to “crowbar” her. Imagine grasping a ballpen with a closed fist. Point the ballpen up towards the fingers and slide the pen in and out. It expands the vagina further than the normal girth.
If girth is king, why the hell does everyone keep talking about length?