Combinations are hard for all of us, but I can offer some advise on a little nuance that can easily be overlooked.
Lots of players like to use a touch of outside.english to throw shots into pockets. When shooting a combination shot, if you use that touch of english to throw the first ball into the contact point of the second ball, even with deadly accuracy, you will most likely miss the shot.
The reason is that you impart side spin to the first ball in addition to hitting it perfectly. That side spin transfers to the OB which in turn throws it covertly off line.
So my cardinal rule for combination shots is no side spin on the CB. Any exception to this must be well considered ahead of time.
More combination shots are missed by a mile because of this leaving the shooter scratching their heads. So be aware.
What he said. I would also like to add that I have more success shooting them with draw. When I have an absolute must make combo I hit it with draw everytime. I also prefer to play position using speed as opposed up taking a perfect angle this seems to bring my percentages up some too.