Originally Posted by Mxfrank

First off, it's a cylinder, not a tumbler. Remove it and check it's condition. You may indeed have a problem with the lock itself.

Here is my technique for repairing ignition cylinders. If you have access to new wafers, you can replace them rather than grind, but I have no idea where you would find them. Grinding has always worked for me:

Lock190

This shows a 201 lock being repaired, other models are variations on the theme. I think the mechanism is best left dry, as long as it's very clean. Graphite is repeatedly recommended here, and I repeatedly explain that everything solid that goes into the lock stays there: graphite deposits will gradually build until it's a problem. An oily spray will attract dirt, but at least it can be flushed out a solvent. And the very worst results will occur if you use graphite on a dirty cylinder. If a lubricant is used, I suggest a dry silicone spray which can be obtained at a locksmith. But no matter what, you want to start with a clean lock.