Well you have one of two dilemmas - either the cow doesn't have any milk, or she won't let her milk down.
When oxytocin is administered, you will see an almost-immediate result, but it's not really something you "SEE". Oxytocin acts in two ways, the first and only one that matters to you - milk letdown. If the cow is truly holding her milk, then you won't be able to get "much" out of the teats. A few strips, and then nothing, but her udder will still look/feel full and you know there's something not quite right. Oxytocin will cause her to letdown her milk and she won't have any choice in the matter - you strip and there will be milk.
On the other hand, if the cow doesn't have any milk... then you're out of luck. Can't make the cow let milk down if there isn't any there. :lol:
BTW, the correct dosage for oxytocin for milk letdown is ~1cc, sometimes more if the heifer/cow is extremely nervous. I've done up to 4ccs on certain heifers. The 10cc dosage is for o.b. use - uterine contractions. Not that 10ccs is really going to be harmful or anything; it's just not neccessary to give that much.
With IM use, milk letdown should be in 1-3 minutes, no more. IV use and it's instant. Wait too long and it's not going to be effective any longer - it does wear off. For that matter, going out and hitting her with oxytocin twice daily and calling it good - isn't. If you're going to give it, have her in the chute and put the calf on a minute or two after giving oxytocin. If you do it like that, and then come to the conclusion the calf is not getting any milk, then I'd go with the obvious - there really isn't anything there.