2.5 Merc 4-stroke won't start half the time

It starts fine at first and then won't restart. I ran it down the lake for 10 minutes to the other end, tipped it up to avoid the shallow rocks and tried to start it. No go. Took it off the boat at home, put it on a wooden horse and no problem starting it multiple times. Back in November I followed the advice given not to choke if the motor has been run within 24 hours.
Didn't work today.

What gives?!!!:(

FYI - been sticking with non-ethanol gas and the oil level is fine.
Also, only shut it off with the kill switch as in the manual.
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