MY Solution: I was having similar problems with half of my products/services showing in the drop-down box but NOT populating the field when selected. Our invoices use a complex "bundle" to track multiple commissions on our invoices. I couldn't access the bundles and I wasn't open to re-entering all the preconfigured data. Here is the workaround:

Begin typing the first couple of letters of the product or service and QBO will quickly provide a contexual popup that contains the missing products/services. Then just select from the "new" dropdown box the appropriate entry and it will populate.

It still adds a step but saves hours of data entry.