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!