You'll need to have the figure in its zero pose but it can be morphed.

When you click the fitting room's "New" button the pop up window gives some options. Under Object you would select the clothing then if you have posed or morphed that in any way DO NOT select the "zero figure" option.

The goal will be the figure. If it was already in its fully zero pose then selecting its "Zero figure" option in the fitting room will not make any difference. HOWEVER if you are fitting the clothing to a morphed figure then DO NOT select that option since if you do the fitting room will ignore the morphs.

What figure pose did you use?

Did you select the option to zero the clothing?

Did you select the option to zero the figure?