Your cartridge has a computer chip monitoring pages printed.   It is called "Chip Information" or "Anonymouse Usage." Once the monitor shows that you printed too many pages, the printer will stop printing. You have three choices:

1 ) Let the printer keep working on refilled cartridges; the ink level monitor needs to be turned off (see below).  The result varies.  Some printer will let you print for 1 or 2 refills.  Some printer will let you print forever.  You will need to keep an eye on the print quality and add ink as needed.

2)  Glue a 3rd party chip on top of your OEM cartridge.  (Product Link)

     2a) For 950/951 Chip, the chip will reset itself and you do not need to turn off the ink level monitor.  There is no limit of how many refills you do.  

    2b) For 932/933 Chip, the chip will not reset itself.  You need to turn off the chip info and the printer will print forever.  You will need to monitor the printout.

3) Buy a refillable cartridge, (Product Link)  These cartridges are not made by HP.  They have two holes, one is the refilling hole, which you unplug and inject ink.  Another one is air vent; you keep it unplugged during operation.  They have chips installed as mentioned in #2.

How to turn off ink information monitor:

For Printers without LCD Screen (e.g., OfficeJet 8100):

1. Press and hold the Wireless and Resume buttons

2. Wait till ink supply icon blink 4 times

3. Press and hold the Wireless and Resume buttons again to turn usage information monitor back on

For Printers with LCD Screen (e.g., OfficeJet 8600):

1. Select Setup from printer control panel

2. Select Preference->Cartridge Chip Information -> OK to turn off usage information.

For New Version of HP printer with LCD Screen

The new HP firmware calls the chip info "Anonymous Usage Storage."  Go to Setup->Tools scroll down, if you see "Anonymous Usage Storage," then turn it off.  If you don't see the selection, it is off already.  You can try a restore to manufacture default and the choice should re-appear (when chip info is on). 

Excerpt from HP OfficeJet 6700's Manual: http://h10032.www1.hp.com/ctg/Manual/c03227497