No matter the orientation the camera should work the same time after time. I make sure the measure the eye piece focus with calipers so I can repeat the focus before sliding on the FTS. Each scope is different. I know external light can make a picture almost completely dim. Also the picture is more dim before tightening down the camera with the Allen key. Make sure the scope FTS adaptor crease is facing down to limit light no matter the orientation, this may help, also the right size - too big will dim your picture. I take it a little further and fill the gap with some black electrical tape, which does make a some difference.
When you have the FTS on the scope and have the camera mounted and it's dim or dark, when you remove the camera does the picture on the camera become clear again through the app?