Hy Buddy, I was also facing this issue but I get the solution of this issue. I am Attaching the link where you may get your solution-
Dell 720 Printer Not Printing Even with a New Ink :
Please have a point of view-
1. On the ink cartridge, you will see a protective tape covering the print nozzle. If you don't remove this protective tape, nothing will be printed on your printer. Also, the ink cartridge contains stickers and labels, kindly do not remove them.
2. Reset the cartridge for the ink. To do so, click and hold for 3 seconds the Cleaning button or "Load / Eject" button. Lift the clamp to secure the cartridge after that, but do not remove the cartridge from your Dell 720 printer. Then shut down the clamp and click the button "Load / Eject" again. If you have any confusion concerning the process of resetting your printer, consult the user manual for the printer.
3. Run an extended cleaning cycle on your printer because the cartridge will be recognized by your printer. Locate your printer's Cleaning button and hold it for 5 seconds. Print a test page after the cleaning cycle has been completed.