Feet slip off flat/cleat hybrid pedals when it's wet I have those hybrid pedals that allow for both clipping in with cleats, and riding on flat pedals with ordinary shoes. 

As I use my bike for cycling around the city, I mostly use the flats. 

What I'm I'm finding though, is that when it's wet my shoes will slip of the pedals. I think it's because the clip in parts are slightly raised above the flat area. 

Any suggestions for mitigating this?
As you have observed, to get good traction for flats (especially in the wet) the pedals have to be grippy. To be able to rotate to enable the release of clipless shoes, the flats have to be slippery. Making the pedals grippy will mean unreliable release when using clipless. 

Whatever you do, do not try to fix it with DIY solutions such as grip tape, screwing plates onto the pedals or into the leading edges (sticking up)  - You will improve the problem you have, but likely create a worse one - you might well find yourself under a B-Train or off the side of a trail and down a bluff when you cannot release in a hurry. 

Solutions are to accept the compromise solution, try the combination flats one side, clips the other which solves this problem (but generates others such as the pedal is never the right way up.), or commit to either flats or clipless. 

Personally I think the commitment to one or the other is the best solution. 

This question (and many others) is worth reading if you want to consider other options.