Furnace blower sometimes won't come on
The normal startup should be a sequence like this:

inducer fan comes on
pressure switch verifies fan operation and flue non-obstruction
hot surface igniter comes on
gas valve opens
gas ignites
flame-proving sensor confirms ignition
blower motor turns on
pressure switch, flame proving sensor, and high-limit temperature switches continue monitoring for abnormal conditions that could develop during operation

All of those things are prone to failure: fans wear out, pressure switches wear out or connecting tubing develops leaks, igniters fail, flame-proving sensors get dirty.
You'll have to create a call for heat and observe how far into the sequence the furnace goes. If the inducer fan comes on and nothing more happens that's a sign there's something truly wrong with the flue or there's something wrong with the pressure sensing. If you hear a click and light hiss (gas valve opening) but never see the igniter glow then the igniter has failed (unlikely: igniter failure is usually permanent, not intermittent). If the gas ignites but then shuts off after a moment, and the controls possibly run through the ignition cycle again, then the flame sensing mechanism isn't working.