I need to get a bottom trace between two via's that currently are connected with an airwire.
However, when doing the trace at the bottom, the polygon does not cede to make way for that trace.
Using Eagle v7.6.0
How to solve?

I need to get a bottom trace between two via's that currently are connected with an airwire.
However, when doing the trace at the bottom, the polygon does not cede to make way for that trace.
Using Eagle v7.6.0
How to solve?

It's been too long since I used EAGLE, so I can't recall a reason (suggest it's time to move to KiCAD maybe : )
Have you tried closing EAGLE and starting it up again, in case EAGLE has corrupted it's internal temporary data (you're using an old version of EAGLE).
As an experiment, you could back up your files, and then delete the ground plane polygon, and then see if you can lay down the trace (and then add the polygon after that).
Also, your traces look extremely thin.. are you sure they meet the PCB factory design rules? Have you loaded any design rules?