As above, maybe you took them off a little soon last year, so she still needs to learn that it's ok.
As someone said above, the relief from being milked is said to be thing that finally convinced the ewe to stand still.
If they're really bad then we'll tip the ewe upside down to begin with, and hold the lambs on until all parties understand... Then switch to standing position whilst holding.
Beware too of what scientists would call 'observation bias '. If you're stood close, the ewe will be in fight or flight mode all the time, and won't let the lambs suck because of the nearby danger.