You  could be pulling up on the shot.  Maybe try to stay low to the ground at the foul line.  That is my classic mistake.
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Don't be afraid of failing, be affraid of not trying!

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I place right foot on 35 , I'm a righty, walk straight to the gutter. Target is right side of center arrow, don't look farther down lane, aim is right side of 6 pin. Mentally I don't have a 10 pin, it's a 6. May sound strange but it was the mental thing of a 10 pin that would kill me, so I (mentally) only leave 6 pins.

Use a plastic ball and unless the thumb swells, my conversion has gone up to 90-95%. Unlike some, I don't speed up, same speed, same release and the timing won't mess ya up.

Just works for me. Good Luck.
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az guy aka: R & L Bowlers Pro
  rlbowlerspro@cox.net

The one trick that has been working really well for me is to envision a black curtain over the lane just past my target.  I force myself to keep my head and eyes totally focused on my target (which happens to be at the dots for me) until after the ball has gone past.  I make it a point to watch exactly what board the ball rolls over at my target point.

I had been streaky regarding corner pins.  I found that when I was on a bad streak, I was raising my head and looking downlane toward the pin too soon, forcing me to either push it right, or reach too hard which would cause me to tug it left.  

My accuracy has gone way up.  I find now that sometimes I miss my target by more than a board, yet still make the spare.

FWIW, I throw straight at all single pins with an old pearl urethane.  Another thing, I keep my arm and shoulder square to my body at all times, but I walk toward my target (and therefore the pin).

The best advice I can give you is to dedicate practice to shooting nothing but 10 pins without regard to scores.  I had major issues with covering 10 pin or other single pin spares that just eroded my confidence until I dedicated time to shoot 10 pins on my first ball in every frame for 1 to 2 games in a practice session.  The focus gave me the discipline to line up, walk straight on the approach & to hit my target line on the 4th to 5th arrow.  I'm now covering 80 to 85% of these spares whereas in the past I was fortunate to cover 50%.  This allowed to work on the single pin spares & to shoot the 2nd ball at other tougher & messier spare shots.

The other things that I've done is to use my plastic ball on all single pin leaves and to have a buddy be a 2nd pair of eyes while we practice.  The latter can help you get feedback on any form breaks or bad habits that you simply can't identify without video or a coach.  If nothing else, seek out a certified coach to work on your spare game.
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Spence Hayashi