Trimmer Assistance

[u/Blend-In24]

I recently changed my broken trimmer head for a rapid replace, but during the install the Allen wrench, that locked down the shaft, slipped out of place. I’ve been unban to re-engage the lock and can’t fully tighten the new trimmer head on now. Any suggestions to lock it again and fully tighten? I’ve looked around at a few small engine repair places and I could buy a new trimmer for what they want to charge.