Rents for mid-month purchase of a turnkey property

[Bill Baldwin]

Quick question regarding normal practice surrounding rents when a tenanted investment property sale is finalized mid-month. I assumed (this has always been the case for me in the past) that when buying a property mid-month that the buyer would receive rent in the pro-rated amount for the number of days remaining in the month of purchase after purchase date. However I recently closed a deal on the 12th of a month and the rents weren't included in the closing. I asked my agent and they said they'd follow up, then came back to me reporting that the previous owner doesn't feel she has to pay this now. Not really sure what my options here are.

Was this a miss by me? Or by my agent and I should have caught it earlier? Curious what convention is and what kind of rights I have to claim my money. It's just under $2000 so not the end of the world, but for someone like myself where that represents a full months living expenses I don't want to give it up when it's something I should he due. 

Thanks all in advance!