I've purchased 3 of these inner tubers and they all go flat after a few hours outside. What gives? has anyone experienced this before? They have removable cores, could that be the reason?

Robot Check

I've examined the inner tube and found no punctures. I pump air in and can't feel any air coming out. I pumped it up to 120 psi and it's holding fine. But i bet when I go back outside the tire will be flat in a few hours.

I've installed all 3 and the same thing happened for all tubes. I've examined the rim too and found nothing unusual.