I think this makes the difference. I used to have the same problem as the OP, but I began to use pliers to pull the string REALLY tight. Now they won't move, and I have even have trouble getting them to move sometimes when I want to change depth.
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I looked at several different brands (Google Image search) and they seem to all be sized at 4-8lb ... 8-12lb ... 12-20lb test line. Not all of them had a bead with each rubber stopper. Colors didn't relate to the line size they're used for, either.
I've got several different ones, and would tell you the brand and line size ... but, unfortunately I don't keep them in their original pkg. but all together in a ziplock bag. I'm pretty sure I got them at Wal-Mart, and I'm "sorta" sure they are Eagle Claw brand. And the pkg did have the line size printed on it, or else I'd never have bought them. Mine came on a white ring, with the red rubber stops & a red bead on each wire.
The bead would probably not be necessary if the stops are used on slip floats with very narrow openings in the stem.
Also, I did notice that some were listed as SS, S, M, L .... and I'm just guessing here, but I would assume that SS=<4lb ... S=4-8lb ... M=8-12lb ... L=12-20lb test line.
... cp :kewl