Ok ..
1.4x TC .. 100mm lens , 15mm or 8mm Lens ...
So here is what I would do : ( Have done )
1.4x TC to body , 100mm lens to TC and then reverse the 15mm lens to the 100mm lens ...
What I have done ...
1.5 TC , 100mm lens with a 50mm lens reversed .. That gave me 3 to 1 Macro ( 150mm to 50mm Reversed = 3 to 1 )
Why do it this way ?
1) Maintain aperture control
2) 3 to 1 Macro
If you 1.4 - 100 - 18 reversed ( 140mm / 18mm = 7.7 to 1 Macro )
That is seriously extreme macro , and you must be photographing really small stuff because your field of view ( side to side ) will be very small .
Your DOF will be small ...
You will need a lot of light to maintain a decent shutter speed to counter movement ...
Remember , the more you magnify - you magnify everything and cut the amount of light ...
You need light ! , you need shutter speed ,
Your description of what you are doing is a little
confusing ...
But if you are lens stacking ? I would suggest starting off a little less extreme ...
Try 2:1 Macro , then maybe 3:1 and 4:1 and see how that difficulty factor is working out . ( Because it gets harder )
I did manage 3:1 , but @ 4:1 it got very Miss rather than Hit ( I shoot free hand )
So possibly a picture of your set up , and camera settings ( ISO - SHUTTER - Fstop )