I found that DS had to be in a position that he didn't associate with boobies - sitting up in his baby-chair, or lying on a cushion.
If he was held then it was no-go.

We had to get him sucking (on a finger or the leg of his teddy (ew!)) first and then put the teat in.

Make sure that you send lots of small (<50ml) bottles to the creche, because they will have to piff the whole bottle if it's not taken.