Manufacturing tolerances can create a situation where a tube ID is at its maximum and a nib or other insert is at its minimum OD . The insert manufacturer would know those tolerances , and should have designed their inserts to work in the worst case scenario , but the best laid plans sometimes go astray .
I`ve had instances where I could insert a nib or finial just by pressing it in with my thumb . Those situations demand some glue help . Another one that can catch you after a significant time lapse , is a finial becoming loose due to the leveraging action of flexing the clip as it is put into and taken out of a pocket . Those movements may only in the range of 0.001 inch , but they will add up .
I use red Loctite on any parts that I`m concerned about . Others use blue Loctite , which is easier to remove if something goes wrong . And of course , sometimes it is wise to deliberately ream out a tube to accommodate very easy insert installation if the wood is prone to splitting , in which case epoxy may be the glue of choice .