One thing I do if I cannot get the foal on any other way is to tie the mare (again helps if someone is with you!) and sit under her, put the foal on your lap, lying down, and hold her until she stops struggling and panicking. Once they are held in a prone position they tend to give up and lie there. Once she is passive, lift her head and just plug her on! Hold her in position, (mine tended to be so passive by this time that they would just allow it) and milk a drop into her mouth- and then again, and again, Eventually she will respond to the milk and swallow, which will mean she is coming out of "passive mode" so be warned. At this point she will, hopefully, actually suck out of instinct (and then explode as it is a HUGE thing to a foal, that first suck) You may have to do this a couple of times, but it has never failed for me- getting the Vet to tube her and putting a few cc's of colostrum down her is a very sound precaution, but I think she will latch on naturally in a bit- once she has accepted you placing her in position she will be more accepting of you placing her head when she is standing.