I had this same issue. Make sure when adding the Journal Entry that you're crediting the depreciation sub-account of the fixed asset and debiting a global Deprecation expense account. NOT the Original Cost or Fixed Asset accounts.

If you don't debit an actual expense account it won't show up in your P&L. See this short YouTube video for how to do this - pay careful attention to the accounts she chooses.

Depreciating an Asset with QuickBooks Online