If a check I wrote shows up in the register, but did not subtract from my QB account what happened?

Good afternoon, @judyls883. 

Welcome to the Community! 

To clarify, have you recorded this check in QuickBooks Online yet? If not, please record that transactions and add or match them together so that the QBO system knows it is there. Here are two guides  to help you perform these actions: 

• Create and record checks
• Match Online Bank transactions

This should do the trick. I'm only a post away if you need me again. Have a wonderful day!