I have read through everything I could find on negative stock problems in the forum, but I am still having trouble.

I am using Stock By Attributes 4.6 on ZenCart 1.3.7.1

I have config>stock>allow checkout set to false, and I set each stock quantity through catalog>products with attributes.

For example, I will have a T-shirt with the following attributes & stock:

T-Shirt Design #1
Small - 5
Medium - 2
Large - 1
X-Large - 8

Someone will be able to checkout with 2 Large t-shirts in their cart and stock goes to -1.
I don't know why this is happening. When I go to test it (on the same product that was just checked out) I am unable to checkout myself. So, it seems very inconsistent.

I have even had times when a certain size was set to zero (0), and they could still check out!

Any clue to why this might be happening? Possibly I have something configured incorrectly.

Any help is appreciated as this is driving me (and my customers) crazy.