The cause of uneaten caecals is usually diet (high carb and protein and not enough fibre) but in your case if the diet is predominantly hay its unlikley to be that unless you've just started a new batch of hay that might not agree with him! If you randomly fed him a handful of pellets particularly excel, than that might have caused it as Excel pellets have a bit of a reputation for that. Are the caecals properly formed or a bit squishy? If not diet related, the production of excess sticky caecotrophs could be a variety of things which would need additional diagnostics.