There are 2 possible reasons for this:
You haven't linked your store to your PayPal account
Make sure your store is properly connected to your PayPal account by following the instructions located here. Until you do, your orders may not be processed properly and may not show up (and may get oversold).
There was a glitch in the system
This is very rare but it can happen occasionally. If you got an email receipt from PayPal and you can see the money in your PayPal account or you can see the money in your Stripe account, then the order has been processed and you should still go ahead and ship it out for the customer.
If it happens more than once, please let us know so we can get it fixed for you.