I am having problems with Zebra printer MDL#ZM400 skipping labels....so if I print 10 lables it might give me 15 lables and 5 of them are blank

Try entering the Menu system and exiting, whilst exiting and instead of saving permanent, cycle around to load defaults. once you have done that make sure the settings are as you need such as Thermal Transfer etc and save permanent. You won't have a problem after that. If you do I would be checking the page setup in the software to make sure it fits the lables.

I agree with Dustproof's suggestion regarding setting defaults. After you do that, you will need to set up your firmware and driver for the correct density, speed, media handling (tear-off vs. peel - labels with gaps or black marks) , etc. If your driver does not relfect the same settings that you have saved in the printer's firmware, the driver will send commands to the printer that will change its settings. Lastly, you will need to calibrate the printer to the media that you are using. An easy way to do this on the ZM400 is to set the power on setting in the LCD menu to calibrate and the head close setting to feed, save these settings in memory and then reboot the printer. This way, you will only need to reboot your printer to calibrate when there is a problem or if you change media types. The feed setting on head close will make sure that your label advances to the the next gap or mark if you open the printhead.
If all ese fails, call Zebra tech support (800) 423 0422 and open up a support case. It is possible that the media sensor or media led is not working properly requiring that your printer be sent in for repair.