How long had the old chain been on?
If for a very long time the teeth on the cog would've worn with it, meaning new chain wouldn't sit on the worn teeth quite properly.
Other than that, check the joining link particularly, but all chain links for damage, or damage to a tooth on cog / chainring.
Can't think of much more than that! Good luck.
@rhb started
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How long had the old chain been on?
If for a very long time the teeth on the cog would've worn with it, meaning new chain wouldn't sit on the worn teeth quite properly.
Other than that, check the joining link particularly, but all chain links for damage, or damage to a tooth on cog / chainring.
Can't think of much more than that! Good luck.