Kudos to wdylanp, exactly the same as my situation--yet another clugey aspect of QBO that I dislike. I nearly created a NEW CHECK (duplicate expense) because filtering the register for "Checks" showed that one check number was missing. Luckily I decided to look at the transactions for the Vendor, and saw it was already cleared and reconciled, but was Expense not Check. I tried to edit the transaction to change the type to Check but QBO provides no means to do so. I think this is absurd, if Expense and Check are so similar why is there no method to change it. And why, if an Expense has a Check#, don't they offer this kind of option when filtering the account register?