As both of the above have touched on, MOST of the time in these cases it is one of two things -- Semen Quality, and more importantly some times is TIMING.
For many years now I have only bred these donors one time and one time only. But it is the right time, not when it is close to quitting time or anything else, it is when the cow is ready. I am very patient too -- you will notice me posting some times at 3 in the morning -- not because I can't sleep -- hehehe!
It will be very hard ot tell you what to think on this with such little info, but 28 does seem like a larger than normal number, we don't try for near that many, and use a very small dose the first time out. Some cows are just very sensitive too -- even with a small dose, we have gotten well over 20 at times.
It also depends on the breeding. Simmi, Gelbvieh, Braunvieh and alot of the Maine blood lines will be more sensitive to the drugs, so back them off more than most Angus cows take to work. Just a hundred things to think about I guess.
If she is still OK in there, back her down and try again but use known GOOD semen andgive her time to get done with her heat -- lots of time!
Terry