Lew Zealand
Helper
It sounds like you're having very bad luck or you may have found a game bug as you're doing it right with the +5 Luck buff. One way to increase your chances of hooking Sea Jelly is to Fish at Ginger Island West with the Training Rod. That Rod only costs 25g from the Fish Shop and hopefully you have that part of Ginger opened up.
However there is a weird detail with how the game generates your chance of Fishing up Jelly: if you catch only Trash or Seaweed but no Fish, then your chances of getting another Sea Jelly are not changed. So if you're only catching those items with the Training Rod, switch to your regular Rod and catch a Fish, and then switch back to the Training Rod.
However there is a weird detail with how the game generates your chance of Fishing up Jelly: if you catch only Trash or Seaweed but no Fish, then your chances of getting another Sea Jelly are not changed. So if you're only catching those items with the Training Rod, switch to your regular Rod and catch a Fish, and then switch back to the Training Rod.