When a Frisky Fox 18 lawn mower shows signs of wanting to start but doesn't fully fire up, there are several additional areas worth checking beyond the basics you've already covered. First, examine the air filter as a clogged or dirty filter can restrict airflow and prevent proper starting. Remove and clean or replace it if necessary. Next, check the spark plug gap and condition more thoroughly - even if it's sparking, it may not be providing a strong enough spark if it's worn or improperly gapped. Consider replacing it with a new one to eliminate this variable.
The carburettor might need a more thorough cleaning beyond just checking the bowl. Stale fuel can leave varnish deposits that clog tiny jets and passages. Try using a carburettor cleaner spray or consider disassembling for a deeper clean. Also, check the fuel quality - old or contaminated fuel is a common culprit in starting issues. If the fuel has been sitting for months, drain and replace with fresh fuel.
Sorry I can't help further.
Mark