Turn signals out on one side only, all bulbs are good

First bypass the flasher and recheck. I've seen "weird stuff" where one bulb was making poor contact, or the common connection in the bulb which should be the ground caused both filaments to be in series. (Same deal as an ungrounded socket) This can cause "fussy" flashers to "not flash" as the bulbs are not drawing enough current

Or take a couple of known good bulbs...........