If you're leaving 9's and pocket 6's, I would tend to think you're coming in too high or at too much of an angle.

But what do I know, you're the coach.
--------------------
Sarcasm Disclaimer:

"This post may contain sarcasm, things that look like sarcasm, and things that might be mistaken as sarcasm. My sarcasm is self-made at my environmentally friendly laboratory and no human or animal was harmed to create the sarcasm found within this post."

Unless, you were harmed.  In the extremely rare case were that to happen, Tough Nouggies!!!
Disclaimer provided by Jorge, Thanks.

Osku,

From personal experience, leaving 9s and 6s on a steady basis, as others have said, is mostly due to too much entry angle.

What I like to do to increase the probability of striking (and reduce the likelihood of stoning pins), is move into the oil just enough to get good skid and increased proper reaction off the spot down the lane, which mixes the pins up very well, whether entering the pocket light or flushing it.

The way I accomplish this is to not give myself more than 7 to 9 boards difference between the starting position of my measuring foot (left foot for me) and the visual target on the lane that I intend to have the ball cross somewhere around the arrows. (Sometimes having more of a foot to target differential is OK, especially when the lanes are wide open, but quite often this is not the case.)

Hope this helps a little.

--------------------
We will be known forever by the tracks we leave.