Hi jamie, yeah unfortunately inset is really pretty sensitive. It has a number of complex steps in it like a series of offsets, and also a merging process that involves merging coincident surfaces.

If you have your geometry to be as clean as possible it can help a lot.

In this case, you've got some surfaces in there that are planar, but they seem to be made up of sweeps or something like that rather than actual simple 4-point plane surfaces, like here:



If you simplify that piece by deleting the front and back faces, and then use the Construct > Planar command on it to cap off the front and back with simple planes, then Inset will work on it:



I've attached the updated 3DM file as well.

- Michael