Sometimes, even when your tenons are tight, the joint doesn't come together properly - there's a gap between the two mating timbers. The problem could come from the tenon being too long for the mortise, but that is easily fixed - simply trim the tenon. But more likely it comes from the shoulders on the tenon stock not being cut properly. If one side is cut at a shorter length than the other, the longer side will close up tightly, but there will be a gap on the other side. If you spot this before glue-up, you might be able to fix it, but if you only spot it after the joint has set, then there's some remedial work to be done.
The fix here is similar in concept to the dovetail repair - a sliver of timber is inserted into the gap. But instead of end-grain it needs to be a long-grain insert if it is to be virtually invisible. Cut a wafer from a piece of scrap that matches the stock holding the mortise. Try to get the grain running in the same direction as the piece being patched - that way the repair can be virtually invisible when viewed from any direction.
Glue the wafer in place and leave it till the glue is set. It can then be carefully trimmed and sanded smooth. A random orbit sander is ideal for this finishing job, because it won't leave visible scratch marks on the two mating pieces. Again, if you have been successful, the repair will be virtually invisible, except under really close examination.