Sometimes rodents that have lived together for a long time can have serious fights and decide they can no longer live together. This is known as declanning. Female gerbils are known for this in later life, if kept in numbers greater than a pair. If declanning occurs in a pair then you may be able to reunite using the split-cage method, but you cannot introduce a single gerbil to an existing pair or clan.
If blood hasn’t been drawn then one of the most common problems may be odour, say if you handled one rodent without handling the other or if there is as strong smell in the room such as if you have recently painted. A dust bath is a good start to try and neutralise this for some species.
Sometimes this isn't declanning, but where a species has been sold to an owner without the proviso that they may not have a good chance at staying together for life. Dwarf hamsters from pet shops for instance, can be difficult as they will be hybrids - and so may have inherited different percentages of traits and not be compatible with each other.