Hillview, keep trying with the sucking especially when she comes by and wags her tail. You could try....,
dipping your finger in warm milk and getting her to suck that,
stroking her rump from the base of her tail towards her hips when offering her milk,
standing along side her or sitting her next to you,
pointing the bottle between your lower legs as if you had an udder hanging from your knees,
Putting her in a pen with her mum or other lambs at feeding times and using the sound and excitement of others feeding to encourage her to get the hang of it.
Good luck, it can be terribly frustrating when they cant seem to get the hang of it.