Are you squirting milk into their mouths when you get them on the teat?
If they are feeling pressured / constrained as you hold them to the teat, they sometimes won't suck. It can be hard to get them in position without forcing them, though! It helps to have one person hold the ewe so the other can concentrate on very gently getting the lamb into position.
Personally I don't tube a lamb which can suck, I use a bottle. In general, once it's tasted milk from a teat, if you put a teat in its mouth and get a bit of milk in its mouth, it'll be away.
Other tips include tickling the top of the lamb's tail, giving it a cuddle before placing it at the teat, getting it on a bottle then bringing the bottle up underneath mum and gently transferring the lamb's mouth to mum, or just pull the bottle out from under so it quests about under mum and finds her teat.
I know how irritating it can be, but gentleness and patience will usually win out in the end. Good luck!