Shaky arms in L sit tuck

I can hold a max L sit tuck for 40 seconds, but even from the start my arms are shaking. I've been doing 3x20s of the the hold, but still I haven't seen an improvement. I can hold 60s PB hold. I don't understand the problem?

Shaking is a problem that usually corrects itself as you develop more strength in supporting the position. My arms shook like crazy the first few weeks of L-sits on the rings, but months later are now as still as stone when doing it.