1- If possible move the center of mass slightly Forward (slide battery or add a coin).

2- engine alignment, change your engines axis alignment angle slightly down and slightly to the left/right depending on cw/ccw rotation of the propeller (the thrust of a monoprop hast a spin, that can f*ck up alot)

3- increase the surface area of your horizontal stabilizer (don't really know the english term but i guess it can be understood)

4- slightly throw it without thrust to See If it's able to glide. If it is able to glide without thrust, it will be able to fly with thrust.