Hey Steve, hey Alex,
if it is the case with your bits (sliding caliper to check), forum members had no problem to find metric collets, see here. Also always use the two wrenches to tighten the collet, avoid to use the red shaft lock button. The latter is only there because the trim router is a hand tool and usually you can only hold one wrench since in the other you hold the trim router, but the right way to tighten a collet is to use two wrenches. You can use the shaft lock for the first turns of collet nut until bit stays in collet, but to tighten use the two wrenches. See also here.
Also numerous workpieces wasted due to wandering bits inside collet were reported because of compressed wood dust inside collet. Clean collet regularly. Industrial spindles have sealing air to prevent wood dust entering the collet.
Then they are 1/4" and inside tolerance. Look at tightening and dust inside collet