There are two sizes of buffer tube, milspec and commercial. The milspec tube has a slightly smaller outer diameter. If you have a milspec tube and a commercial sized stock then you would get wobble.
Typically there's not any markings but if you have a set of calipers just measure the diameter of the tube. If it's about 1.15" diameter then it's milspec, about 1.17" is commercial.
Hope this helps solve it for you.
EDIT: Here's a picture to illustrate what I mean:
