There are three main reasons for initial tubeless “failure”:
- the tape rim is fitted incorrectly or has been damaged.
- the tyre isn’t seated properly.
- See point #1
Often when the “valve” leaks it’s really the tape that’s been damaged and the air has just found its way round to the valve to escape.
Tubeless can be frustrating with an identical tyre/rim combo working on one wheel and not on another - I’ve had tyres stay up for several days without adding sealant yet the same tyre on the same rim won’t stay inflated for an hour. In those cases I always go back and strip off the tape, clean the rim and apply new tape.
Once I’ve got the tyre on and inflated then I go for a ride, only has to be half an hour or so, to help work the tyre bead into the rim and get the sealant into all those nooks and crannies. Do that and it’s then weeks before I need to check the pressure again.