Make sure your nozzle is clean before you start, after that you must lower the temperature off the filament with 10degrees. And then you try something, just a test file. When you see nothing comes out, you push from the back the filament in to the nozzle, normaly then you push all the stuff that is stuck out of your nozzle.
It’s possible it’s burned and it will smell, but it should work.
Make sure it’s not the feeder that’s the problem rather than the nozzle being blocked. My experience with bronzeFill has been that it’s less likely to block the nozzle than standard PLA, not the other way round.
If for any reason the feeder has not been pushing the filament through efficiently and has been grinding the bronzeFill then there will be a lot of bronze particles “coating” the metal hub of the feeder which creates problems afterwards for either bronzeFill or PLA. These bronze particles prevent the hub from gripping the filament correctly and hence pushing it efficiently into the nozzle.
Cleaning out the feeder periodically is a piece of regular maintenance that I would recommend regardless of what materials you’ve been using. Unfortunately the feeder on the UM2 is not the best of designs, hence why it’s the most re-designed part for it on YouMagine.
I never had problem with bronzefill. It’s all about speed and temperature. You must know more about material. At the first clean your nozzle, use aceton, water, burn nozzle, hot wire. Try again if is not blocked. If not, see official website which temperature is correct to use and test it with printer. Jammed nozzle means bad speed with bad temperature or material flow. Ultimaker is powerfull printer and you are able to change almost everything during print. Try it.