The possible reasons are many.

Is this a recent development, or was it this way since new?

Did you try a new plug? (try an OS-8)

Are you using nitro in your fuel? (they really like 5-10%)

Have you overheated it?

Does it have good compression?

When primed, and glow is heated... if you firmly grasp the prop in your hand, and
slowly pull it through compression, can you feel the "kick"?
If so, it should flip-start. If not, it's probably a bad plug, bad fuel, or in need of overhaul.