While there can be other possible causes this is likely due to a sensor problem that detects the index hole, which indicates the start of a label. You usually test for this by using the manual blue lit form feed button, to see if the labels stop reliably between labels.
Unfortunately you can't really clean the printer, especially the index sensor without opening the printer. To get inside you will need to remove the front cover. It just snaps off and snaps back on, but you will need to be careful. To do this, remove the power and open the lid as if loading labels and remove any labels. You will find two small retaining tabs right above the slot on either side where the labels go in the slot. Lift up carefully in the middle, just enough to clear the tabs and pull slowly forward and down to remove the front cover.
Once the cover is off you should be able to look for anything stuck to the rollers or obstructing the slot or other areas. Use a Q-Tip with isopropyl alcohol to clean the rollers. If a roller has gotten a real mess on it, carefully use an Exacto knife to scrape it and then use the Q-Tip and alcohol again.
Looking in from the front with a flashlight you also will notice the optical sensors at the front left about a half inch over, above the slot, right behind the tear bar. Make sure these are clean. Dirty sensors will cause various labels to skip or not stop in the correct place.
Also check that the rollers are evenly engaging and that the release lever is going all the way back. If rollers are uneven and the labels drift to the right, the sensor will not properly detect the sensor hole in the labels. This can also happen if the flanges on the roll holder are not tight, or the right hand paper size guide, at the entrance of the printer slot on later models, is not adjusted properly.
To put the cover back on, start at the bottom and carefully hinge up to click shut at the top. Then retest using the form feed button.
If you still have problems come back.
