Hyster 50 forklift won't start cold. Starter motor is fine. Occasionally if will start, then runs fine all day for additional same-day starting, but the next day, it won't start again... temperature in about 45-50 F where it's stored. The air filter is clean, the fuel tank is full of propane. Help... it's driving me nuts!!!! Thanks.

Optional Information:
est. 2001 hyster Hyster 50, forklift s50xm--- 11839h 4 cylinder

Already Tried:
swapped between 2 lpg tanks. pushed male part on connector to tank inward to check if spring was good going to tank and seemed ok. pushed button on regulator/diaphram 30+ times, then cranked. cranked while depressing throttle pedal. released pressure in line by disconnecting from tank. checked air filter. checked all wires i could see. checked plug wires. tried to crank for between 4-8 seconds. typically this machine is only fired up once a week. the last three months only 2 times a month. recently before this problem, forklift has been fired up 1+ times a week. left for 3 days. then no start. tried again 4 hours later. after a lot of cranking, fired up. usually next day ok after that but could not start this time at all. lpg tanks are full for sure. no frozen lines. machine not ran very long at a time. no under seat sensor for starting either, so i know that is not it. 11,839 hours on machine. professionally serviced three months ago for oil leak.