I have not used T-58 but do brew a lot Belgians and Saisons.
One thing I have found with these yeasts is that they often take off fast and slow down, but continue to work for a long time. Let it go for a couple more weeks. Depending on what temp you are at now you may want to get the temp up to the upper end of the range. That often helps.
Here is a great quote from Brew Like a Monk.
"Let the fermentation finish, perhaps at a higher temperature. It can often take as long to get the last few points of attenuation as it does for the first 80%."
I routinely give my Belgians a lot longer time in the fermenter than brews like IPA's etc.