Toomany, maybe it is just that you have so much on your plate? It can be hard to anticipate excitedly the arrival of more work! (Sorry, that sounds dreadful - of course you are going to love and bond with your baby, but given you've BTDT a few times already, you are more prepared than most for how much time and energy a new baby takes.) Plus, being in your final weeks, you must be really tired.
I would say that even if the mummy-joy doesn't kick in when you see bubs at the scan, then it will once you have him in your arms.