There are a few reasons tokens can go missing.
You sent them to someone else or another wallet. This is the most obvious reason. This can also occur when you sell tokens and someone buys them. You probably remember if this happened.
Memo token balances are incorrect. SLP token balances sometimes get out of sync on the Memo website. You can try contacting us if you think this is the case.
You loaded your private key into another application. Not all applications support the SLP token protocol. If you loaded your private key into another application that swept your bitcoin or created a transaction, it may have burned your tokens. If this occurred then your tokens are destroyed and cannot be recovered. When exporting your key on Memo we warn using it for backup purposes and that only experts should load their key into other applications.