Hi Samsam,
Ok, this is going to be long-winded but here goes...
Some guinea pigs will take to litter training and some won't no matter what you do, that's just their nature but, there are some things you can try that will work for some pigs.
During floor time (use longer floor time for this), try covering the whole area in newspaper or other material you want to use for their litter box/litter tray/litter area. They will happily poop and pee all over the place. Do this for a couple of weeks for long periods (you can simultaneously do the same in their cage although this doesn't necessarily improve your chances of a successful outcome).
Slowly and incrementally, you need to reduce the area covered by the newspaper etc. (we use toilet paper but you can also use various litter materials). Start by reducing it by a third. Do this for a week or so and you should notice your piggies pooping and peeing on the newspaper and not the uncovered area (hopefully) if this works, redcue again and again until you only use the material in your litter box/tray. Once this is done successfully, you can introduce the litterbox/tray into your cage. The should now associate the litter box and newspaper (or whatever) with pooping/peeing.
We have had great success with this technique for peeing. However, pooping is much more difficult just because there's so much of it and, as you've probably noticed, sometimes the piggy doesn't even notice when they're pooping.
What you will have to do is to do regular and frequent (at least daily) spot cleans, removing and replcing the material in the litter tray. The reason is that, if this gets overly dirty, the piggy will not want to use it and will begin to use any other area of the cage. This is probably why they're using the hammock right now.
I would also suggest removing the hammock during this process and putting your litter tray in the same spot. If the piggy likes this spot then you can continue to encourage them to stay there but use the tray.
Hope this helps
