I recently repaired a fairly long (5mm) tear in the sidewall of a Bontrager mud-x, using a regular mtb patch wasn’t enough, the patches are too thin so when the tyre was inflated it bulged out lots.
I installed a car tyre patch over the existing patch, these ones are much much thicker, it still bulges a bit but has been holding air (tubeless) ever since, that’s about 4 or 5 rides now.
As someone said, it’s very important to install the patch correctly, clean the area well (isopropyl alcohol will help) roughen it up nicely, let the glue dry (so it’s not tacky) before you apply the patch, clamp the patch on to the tyre too, I used a pestle as I don’t have any clamps 😀