A different type of wheel magnet may help. If you are using one of theose flat plastic types, get a metal "button" type with a set screw in the back. They stick out further.

I have an Astrale mounted on my trainer bike. The magnet ( a button-type) is mounted right at the third spoke crossing about 1/4 of the way from the hub to the rim. The pickup coil is fastened to the chainstay pointed toward the rear of the bike and rotated in a bit to get the proper gap. I have no problems getting a strong reading