Weird Front Lever problem?

Hello

I've been training the front lever for about half a year now.
Currently I'm able to hold 4 sets of 1 leg straight and other leg bent (foot touches the knee of the straight leg) for 8 seconds

If I try to do a full front lever on a low bar (close to the ground) I'm able to pull a good one for about 3-4 seconds

However, if the bar is high (regular pullup bar) I can't, not even for a second!

I guess aiming for longer hold sets and throwing some adv.tucked pullups will eventually get me to a full front lever, but I'm really curious about what's the science behind the current situation haha.

Does anyone know?

Front lever on the low bar takes less effort to get into than on the high bar. 

If you do one close to the ground it is likely that you have not fully locked out your arms, since you will go into FL starting with bent arms. This recruits your biceps and makes the move easier.

But if you try to pull into one from a deadhang/ lower from inverted hang your arms are more likely to be locked out. This makes the move harder. Additionally pulling into the move (assuming you're using a bit of swing) makes the move harder to 'stick'. That's my opinion