Are you using the re-fill kits?
some of the JR refill kits come with something called cartridge flush (clear purple liquid) get a small dish and pour some of the flush into the dish, next place the cartridge so the print head rests in the flush solution and leave it there for a while.
If that doesnt work, or you dont have any cartridge flush, get an old sacer/small plate, ie one that you can balance the cartridge on the edge with the printheads in the 'bowl' of the plate.
take the cartridge off the plate, and boil the kettle.
pour the hot water on the plate to create a shallow pool, and then place the cartridge on the plate so that the print heads are in the hot water, After a while you should see some ink flowing through the heads into the water, remove the cartridge from the water and gently dab it on some soft paper ie kitchen towel - (dont rub the ink heads!)
next place the cartidge onto a dry clean plate for a while to make sure the ink has stopped dripping through.
If its still dripping out, after a minute remove the cartridge from the plate, dab it again, cleare the ink from the plate and put the cartridge back on the plate at this point verything should be OK, and you should be able to put the re-filled cartridge back in your printer, as use as new.
As soon as you get a low ink warning you need to re-fill the cartridge to prevent it from drying up.