The surface alignment option relies on the objects having been creating with the Y axis pointing up, which is how things are expected to be in Poser. If models are modeled in a different axis system, like it is with 3DSMAX (Z points up), I have to use the export options to convert the axes to Poser standards, so Y points up, X to the right, and Z towards you. When we enable surface alignment in the plugin, it will align the model to the surface's Y axis (normal vector), so if the geometry has different axes orientation, things will flip around.
Another thing is that objects need to be modeled as "sitting" on top of the origin (0,0,0). If we model objects with half the object sunk into the ground plane, or floating at a random place, it will not align correctly in Poser. Depending on what 3D modeling software you use, the process to center objects and reset the origin point may differ.
For example, if I model a prop in 3DSMAX, and then move it to sit on top of the ground plane afterwards, the origin will still be incorrectly placed. We have to "reset x-form" and then move the origin to (0,0,0). Otherwise, whenever I "reset" the object, it will sink into the ground again, because it was originally modeled in that location.
Hope this helps.