Chris Curl started a sharpening thread some months back. Worth taking a look.
http://www.woodworkingtalk.com/f11/share-your-sharpening-secrets-48401/
Too many variables to attempt to provide how to troubleshoot all potential problems with sharpening.
Whatever method you use for sharpening, ensure you get a burr on the back side of the blade for each grit. The burr gets smaller with the higher grits.
If you do not get the burr, it means you did not get the grit to the edge of the blade. Been there, done that. Had a nicely polished blade but it did not want to cut. I had not been paying attention to the burr. I do now.
I happen to use a diamond lapping plate to start my sharpening of the plane restorations then complete with wet-dry sandpaper on a granite slab. I do use a Veritas Mk II honing jig.
Once I have sharpened a blade, I may hone the blade with a hand held credit card diamond sharpener or I may go back to the sandpaper and granite slab with the jig.
In addition to the blade sharpening you should also flatten the front edge of the cap iron so you have good contact with the cap iron and blade across the entire width of the blade. If you do not have good contact, shavings will get stuck and cause the plane to skip.
You should be able to get thin consistent shavings on soft wood. I use a piece of 2x4 I think spruce which has no knots for my testing. I plane one of the edges.
I am normally setting the blade to get very thin shavings.
Another aspect of tuning is the distance of the front of the cap iron to the edge of the blade.
When I first started to use hand planes, I thought the blade did all the work. I thought the cap iron was only to prevent the blade from flexing.
The cap iron does work to stop the blade from flexing, but it is incredibly important in how the shavings cut. Not intuitive.
Over time I read more, including an old book "Planecraft" by the folks at the old Record company G.W. Hampton and E.Clifford. Sub title "Hand planing by modern methods".
I bought mine from Woodcraft. I read that it is out of print. It was reprinted by Woodcraft.
The authors used to work for the old Record company. First published in 1934.
This is a well written book. Lots of useful information.
The section on plane adjustment has the following recommendations for the distance of the cap iron from the edge of the blade.
For rough work, cap iron 1/32in to 1/16in from edge of blade.
For finishing work, cap iron 1/64in from edge of blade.
For hard woods with irregular grain - as close as you can get it to the cutting edge.
Useful, although not quite so simple. Try closer and if the blade is sharp but not cutting pull the cap iron back a tiny amount.