Building queue

I have 4 slots in my building queue that mostly works as it should, how quite often when I try to to add a fourth item to the queue all items in the building menu are greyed out. If I logout and then login again all queue slots are available also if I stay logged in and wait 5 to 10 minutes the queue slot will sometimes become usable.

Edit 1. I now have 1 iron mine and 3 gold mines being built but only the iron mine and 2 gold mines are shown in the build queue

Edit 2. The Iron mine has just completed and I now have 3 gold mines shown in the queue

sounds like an optical thinghy then

Lets bump it down the list :P