I see two immediate issues that could cause this.
1)The most obvious problem is a permission issue, does the second user who is having the problem have exactly the same permissions as the first user? If not that is where I would check.
2)Solidworks and Enterprise each have a reference list for a particular assembly, these lists should be identical but there are a few circumstances where they are not. Depending on the order your checked in the assembly its possible that the Enterprise doesn't know that the o-Ring is required for that assembly. This problem is not always obvious because if someone has used the o-ring in a different assembly when they open the problem assembly it will work because the o-ring is already in the Vault.
I would guess the first problem is far more likely.