Two problems related to transmission not go into gear have been reported for the 2015 Subaru Impreza Wrx. The most recently reported issues are listed below.
Driving on the freeway during rush hour and tried to downshift (manual transmission) from 6th gear to 3rd since traffic slowed down but the shifter won't go into gear and I was going up hill. I tried 4th, 5th, 3rd again, 2nd, and I'm slow enough to try even 1st and still won't go into gear. Fortunately I had enough momentum to get to the shoulder going up hill in the freeway. I had it towed to the dealer and the service rep could not shift it into gear also. He was able to put it into the service bay by turning off the engine, putting it into gear, turning on the engine and driving it. At that point it shifts just fine and was told to let them know if it happens again. This is a big safety concern for me if it happens again since I won't be able to shift/accelerate to avoid obstacles or prevent accidents. There are times when I've felt that it's been a little hesitant to get into gear.
Once, three months after purchasing, and a second time eight months later, my vehicle suddenly would not enter gears from neutral. In the first event, the car was still warming up, driving quickly. In the second event, the car was at running temperature, driven for one hour, driving conservatively. In both situations, I had to pull over to the side of the road and call a tow. ~1 hour later, the vehicle worked fine. No burnt clutch smell, no change in the feeling of the clutch/gears. Dealership could not replicate the problem, corporate is denying the problem.