mine -100 flies straight. as shown in manual - you turn it on in MAN mode, and then you can switch it to other modes.  if i turn it on in MAN mode it'll level wings, and will maintain current attitude. maube you engaging AP with nose-down?? then it'll hold that pitch and you'll go down.

also look at the gps panel. check that avionics is on then you engaging autopilot.

Have a nice flying )