They are credited with point-blank means of defeating the Romulan cloak on the Defiant, using antiproton probing beams. Not infallible means, though, but usually effective as long as the prey doesn't actively try to slip away. But the Romulans probably sold Starfleet the crappiest possible cloak, and Sisko might have used better cloaks on the mines, perhaps ones designed by Starfleet's Klingon allies. We know Klingons are master cloakers - they have infiltrated Romulan space on occasion, even deploying agents to the Romulan homeworld ("Unification")!
The Dominion is later credited with a sensor array that can detect cloaked ships "over five sectors", too. Supposedly, that's a completely different technology, and supposedly, our heroes specifically worry that it works against the cloaks of their Klingon allies. Plus, the Dominion has cloaked mines of its own, sort of: it knows how to dip mines in subspace, as in "Siege of AR-558".
However, O'Brien argues that while the Dominion can detect cloaked ships, the small size of the mines would make all the difference in the efficacy of cloaking. Apparently, he's right...
Timo Saloniemi