Heavy eyebrows / shelf above eyebrows. Can this be fixed with more Botox? (Photo)

I'm sorry you are unhappy with your Botox results. Botox effects will gradually fade over approximately three to four months. You will return to the look you had prior to treatment. Always see an expert injector for optimal results.

Thank you for posting your question here let me start by stating that my answer is based on the limited information and pictures provided. for a more detailed and bespoke plan please consult with a plastic surgeon in person.
adding more botox will unlikely take that area down especially if you already have no movement in it and you might risk dropping your eyebrows. The other issue is that the ridge that you are seeing could be the result of your bone having a ridge and not necessarily your muscle which I cannot tell just based on the pictures. If it is your muscle then over time and with regular botox injections it might weaken and attenuate but if it is your bone and it's bothering you then you can consult with a plastic surgeon who can potentially shave down the bone to give you a smoother transition.
I hope this helps, best of Luck!

Your eyebrows seem a little low centrally but the ridge above that is likely to be bone. We shape and remove that regularly along with a brow lift. Very beautifying!