Hard to say, but if you could convert it to subds before you merged the verts, you may find that Deleting the History will help.

Failing that, open the script editor, and look for the error message that pops up after you attempt to convert. I commonly get stopped by nonmanifold faces or because I use a shelf button to do the conversion, and the number of faces specified in the shelf command are not enough for the model I'm working on.
S.