People (including myself) struggle with the back foot in yoshinkan styles. I'm not going to go deep into the reasons for it, but I will share some tips.
Before we go any further, are you keeping the front foot down as you slide? If not then that needs fixing first.
The key to keeping the back heel down is to use it to push from when sliding forward.
Once you have started to move and extended to a deep position, if you are projecting forward correctly your back foot will start to slide. Keep your upper body locked with your lower, extend your back leg so the 'blade edge' of you back foot makes contact with the ground and continue that feeling through your body and out of your head and fingers.
Make sure that your feet stay in the same '10 and 2' position from basic stance or you will start to push your back knee in the wrong direction.
Just a few pointers. I'll try and get a vid up soon.