Maybe this one will work: LOWER THE PRICE!!!!!

Hi Glenda,
I took a quick look at MLS condo listings in Burlington and didn't see any co-ops. (I assumed it was in Burlington because that's where you posted your question.) So that may be the first issue. If people can't find it, they can't buy it. MLS feeds most of the other sites that buyers use, such as realtor.com. Plus most buyers these days are using a buyer agent, who will rely on MLS.
Maybe I have the wrong town. The reason I looked in the first place was to see if anything struck me as a reason why it wasn't selling- but without knowing any more about it, I can't tell for sure. The 3 most common issues these days are: price, staging ,and/or condition. Hope this helps!

Deb

co-ops are not as prevelant in MA ( I assume your parents is in MA) they are not well understood by the public and many in the real estate community. In NYC they are the predominant form of ownership.
Do you have a realtor who is familiar or has sold in this complex before? Perhaps you can find out from someone on the board there, who has been selling them.
Good Luck