Cow aborting- What's the reason?

Some friends have a cow that had a successful pregnancy and calved in 2010.

She cycled and was successfully bred both natural and AI to different bulls.  First try was the same bull that sired her calf in 2010.

She has since aborted three times after 70-80 days. (At least two different bulls)

I suggested lepto-vibrio-bangs, but all vaccines are up to date.    No other cows have experienced problems so feed has been ruled out.      Vet thinks possible a progesterone deficiency with regular PG shots, but these are impractical. 

What is going on?

I don't know about the cause (we lost a 70 day old pregnancy this last summer, and only caught it because we had her in for sexing the calf, but we were told it was lost because of the heat), but what forms of progesterone are used in cattle? We had a very expensive mare many years ago that could not maintain a pregnancy, and had to be put on a daily oral of progesterone; we just top dressed her feed every day. If I remember correctly, she did not need it the entire pregnancy, just up to so many months until the pregnancy had become well established and her bloods came back that her natural progesterone was high enough to maintain the pregnancy without the artificial progesterone... just a thought.

There are a number of viral diseases that can cause early abortions. If he's sure she's conceiving and then aborting, and the vaccination program is up-to-date, it's also possible that her system hasn't responded to the vaccinations. I say it's POSSIBLE because giving a cow a shot doesn't necessarily guarantee she's protected. There's a lot I don't know about this cow, but she'd have to be something special for me not to thank her kindly for her services and give her early retirement.