This is a very common problem with servo reverser's and exactly why I do not use them. I have seen first hand one that locked an elevator half full down in flight.

The other thing is the servos you have are not programmable. If you have the ability, put both elevator servos on separate channels and depending on the radio, you can either set dual elevator servos or mix the two channels, then reverse whatever one needs it in the radio.

If you let us know what TX/RX you are using and depending what it is, we may be able to help you attempt this