I found our Robo would never eat a whole tiny bit of veg but he’d have a nibble out of it
. Check to see if it has been nibbled. I agree he is probably hoarding at the bottom of the cage in numerous places. Our robos food bowl never seemed to go down yet he clearly was taking food as I found quite a large hoard in one corner at the bottom of the cage. They do eat really tiny amounts.
One thing I did notice is ours tended to ignore hamster mix if it had been sat there for a day and only like it if just put out (smells fresher maybe?). So I used to change his food every day - you waste a bit but at least they take it!