A hot surface ignitor should not be cleaned, you will crack it, then
you will need a new one. If it is spark ignited, well I never found a
need to clean those either. Sounds like low air flow caused by:
1) Restrictive air filter.
2) Restrictive or undersized duct system.
3) Registers turned of in parts of your house you are not using.
4) Blower speed set too low.

Other possibilities: Limit switch improperly set, heat anticipator on
thermostat improperly set.

Stretch