Have you selected the iron on transfer option in your printer's preferences area and are you using the correct print media?
Here's how to do it. This is taken from the User Guide for your printer
To reverse an image for iron-on transfers
1. On the File menu in your software application, click Print.
2. Make sure the product is the selected printer.
3. Click the button that opens the Properties dialog box.
Depending on your software application, this button might be called Properties, Options, Printer Setup, Printer, or Preferences.
4. Click the Features tab.
5. In the Paper Type drop-down list, click More, and then select HP Iron-on Transfer from the list.
6. If the selected size is not what you want, click an appropriate size in the Size list.
7. Click the Advanced tab.
8. In the Features area, select the Mirror Image check box.
9. Click OK, and then click Print or OK in the Print dialog box.
Hopefully this is of some help