I appreciate you for getting back to us, DRAccounting.

First off, let your client configure their printing setup. It is to determine what causes the system to print a blank check.

Let them make sure that their printer and the checks are compatible. Then, try printing a sample check.

If the same thing happens, let them contact our Phone Support agents to have this investigated. 

Lastly, let your clients use the pre-printed checks. This way, the memo will be included when printing the checks.

I've added this article for more information: Print a check in QuickBooks Online.

I'll be around if there's anything that I can help. Keep safe!