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.