It can take a while to become accustomed to new foods. If they were pet shop piggies then they are unlikely to have been exposed to much so will need time. Keep trying with the same item for a while. Lettuce, cucumber, bell pepper and coriander are safe daily foods. Everything else needs to be in moderation and rotation. cabbage and any member of the brassica family can lead to gas so don’t overdo those. Spinach is high calcium so should only be given a couple of leaves once a week. I tend to only feed mine veg which I buy for us and give a bit to piggies - the only thing I buy specifically for the piggies is the coriander!
A thin slice of spring greens once a week is fine though as it provides a balance of other nutrients.
