ProShares is not an up and up company in my opinion. They may not actually short the stocks but use puts, leverage, and other methods to simulate double shorting. Their expenses are high so even if you are right about the direction of a stock you may not make any money. I wanted to short the ultra-short China FXP at 140 a few days ago, and would have been up over 40% now, but it’s not available for shorting. Shorting a ProShares short would be a good way to go long because then their high fees work in your favor.
You could have simply gone long FXI.
Not being able to short is not the fault of ProShares. It probably just meant that your broker had no shares to lend.