Sales Tax Checked But Is Not Charged by Ebay

[wes5265]

I have a sales tax question.  I live in Texas and need to collect sales tax for items shipped to a Texas address.  I have set the seller preference to 8.25% for Texas shipments and checked the box, but the two recent purchasers in Texas have either not been charged or did not pay it.  Does state sales tax only show if I send out an invoice?  I appreciate any help with this issue.