As I understand it, the rumen isn't fully developed until about 6 weeks, which is why we are recommended to keep them on milk until then. So although a younger lamb may be eating creep, it won't be getting the nutritional value out of it.
My own experience is that older (3-4 weeks) lambs who won't take the bottle don't thrive as well as those who do. So I would persevere. With our lambs, it really is only a very tiny minority who won't ever take to the bottle. However, several days to accept it is common; a week not uncommon (with older lambs) and I had one took 10 days, I think.
If you can steel yourself to do it, they'll come around sooner if you keep them a little bit hungry. I have just had to get a month-old pair of lambs onto the bottle for a neighbour whose triplet mum came down with mastitis. The tup lamb took the bottle on the second feed, his sister was more of a challenge. For the first three days they had ad lib hay and a good helping of cake twice a day. Sis was still not taking the bottle so the next day they had just a tiny bit of creep once - but still ad lib hay. Guess what
. You got it - fifth day, she suddenly liked her milk
. I still have to catch her and put the teat in her mouth but she guzzles like a good 'un once she has the taste in her mouth.
Oh, and another tip - feed other lambs in front of the awkward cuss. Let it be hungry and running about you and the other lambs as you feed them. Let it watch them feed; most lambs will start to sniff about the lamb that's feeding and it helps to give them the idea that there is food here, that it comes from you and those strange hard plastic thingies.