My mare is having trouble letting her foal suck?

I have a few mares and foals.The last mare foaled early Monday morning.Its her 1st foal,everything was fine until today.Everytime the foal goes to suck the mare wont let him,she keeps walking.She walked around the paddock,sometimes breaking into a trot for 15mins.Eventually she does give in and lets him suck but tucks right up,indicating to me she is sore.I dont think her milk is flowing down proper yet.I held her in the paddock today to let the foal suck,which he did,but it looked like he wasnt getting much.The poor foal is tired from all the walking,and the mare is sore,her udder looks normal, what can i do ?