This year so far I've had 3 I was sure were never going to feed and would die.

All three now love me to bits, two are the greediest guzzlers you could ever wish to meet.
It will come. Promise.
Yes try different teats. Softer, harder, ones that dribble, ones that don't. Large bore, small bore.
Use different positions - lamb on ground, standing; lamb on your lap; lamb with rear end hanging off your leg.
Move the teat in and out of its mouth - some like it right in there, some just the merest tip.
Try different temperatures too - some like it blood heat and some much cooler.
And don't make all your interaction with these lambs be about forcing food into them, be it by bottle or by tube. They've lost their mum and they need
love. Spend a bit of time cuddling them, stroking them, rubbing your chin on the top of their heads. Talk to them kindly. If you can tap into their reaction to their mother's nickering and nuzzling, they may just suddenly lift their nozzles and start questing for a teat

At their age and stage then yes, I would also tube feed if they won't suck. Every feed I would try with the teat; once you are confident they will swallow when milk is in their mouths then don't be frightened to dribble some of the milk into their mouths and hold their chin up so they swallow. Then if they are clearly not going to suck, tube - but only enough to keep them alive.
They
will suck, honest they will. I wouldn't be so confident if they were still with mum, but once they're taken off their mother, they
will come round.