Pausing the game and demolishing a conveyor results in stuck conveyor items that do not reach their destination.
The conveyor items that were stuck were not on the conveyor when it was demolished, however I assume they were queued to go to their destination. When I resumed the game those items were taken from the stockpile and stuck where the conveyor used to be located, as indicated by the yellow line. You can also see there are three empty slots for windows on the fit window stockpile where these stuck windows were headed.
To resolve I had to rebuild the conveyor over those stuck items at which time they were able to be transported to the appropriate locations.