I'll help you fix this, Kathy710.

Once you assign an expense category to a vendor, QuickBooks should automatically populate that account when you create expense transactions. It could be that the stored cache and cookies in the browser are the root cause of this strange behavior. To isolate this issue, you can try creating a sample expense transaction using a private or incognito window. Here are the shortcut keys:

• Ctrl + Shift + N for Google Chrome
• Ctrl + Shift + P for Firefox & Microsoft Edge
• Command + Shift + N for Safari

If it's now working well, go back to their regular browser and then, clear its cache to delete the accumulated junk files and start over fresh. You may also switch to other supported browsers to further isolate the issue. 

In addition, I recommend reading this article to learn how to record, edit, and delete expenses in QuickBooks Online: Enter and manage expenses.

Don't hesitate to drop a reply below if you have any other concerns or further questions. I'll always have your back