If i remember right, the way to check for proper down thrust angle is to fly at 3/4 throttle inverted. then cut power if your plane gains altitude then you had too much up thrust.
tail not coming up on take off...there are a few things or combination of things which might cause this too slow a take off, tail incidence wrong(down) wing incidence wrong (up) cg tail heavy.
the aircraft gaining altitude when power is applied is normal
since your changing the thrust angle be ready on the elevator on take off, and you might have to have some back pressure on the elevator with power applied
Good Luck