Maybe they just aren't for your rims.
There are two sizes of Veloplugs
The Yellow Veloplugs fit rim holes from 8.5 to 9.3 mm
Red Veloplugs fit 8.0 mm rim holes.
Red or Yellow? What size / colour should I use?
Unfortunately for many rims this can not be said with certainty since the hole size may vary from batch to batch.
To be sure please measure the hole size of your rim before ordering
https://veloplug.com/en/