I would also add ageing engine / maintenance issues into the mix.
In autumn, several of our aircraft (C152) became progressively hard to start, and it became next to impossible in winter - heavy priming, throttle pumping, all the BAD things we had to do to get them going at all... and when really cold even that stopped working and we could not fly.
We leaned on maintenance, they replaced the primer nozzles, and hey presto, 2x primer pumps and they started every time for the rest of the winter. 1x would get it to fire, but would not always sustain it.
Also - is anyone actually following the POH procedure for priming a PA28? Read it an be surprised... I have yet to meet a pilot who uses it, everyone just primes 3 or 4 times and cranks. The POH says crank, if no start THEN prime ONCE and crank again. Works very well...