Well, I would say you are rushing the step where you melt the glass down on the cap. And you likely don't have the cap itself hot enough to make this step flow.
When your jelly is all done, tentacles done, cap on, and you are
ready to put your top clear glass on it, heat the cap up just a bit,
not enough to make it soft, just hot. Heat your glass for the top,
but keep the cap part hot, don't forget it. I like to make a big gather
of clear for the top and slightly maria it , heat the center and then
join but don't try to get the glass all the way down on the cap.
burn off your clear for the top, and then heat from below, aiming
your flame up under the edge of the clear, turning all the while.
I love to use my butter knife (graphite tool) at this stage to
push the glass down little by little as I continue to heat and turn it.
I have used a metal tool to do this, if you have something brass
that would work ok, but beware metal marks, that's why I love
the butter knife I got from Blastshield. Give this a try and report back.
Your jellyfish is lovely, love the dotted cap.
Here's a recent one from me.