I've read sealant can get into valve cores and stop them working. That might explain the first issue.
They were fine, air was coming out around the stem.
You cleaned the rims after removing the old tyres. Perhaps this disturbed the rim tape or valve enough to stop it sealing. Or the new tyres were different such that the amount of tape was not enough to hold pressure.
@Dammit started
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I've read sealant can get into valve cores and stop them working. That might explain the first issue.