There are two issues: UK and US nationalism preventing companies to shift finished products to the EU in case of production troubles, while the EU has been slow

There are two issues: UK and US nationalism preventing companies to shift finished products to the EU in case of production troubles, while the EU has been slow to impose restrictions in the other ways. This meant that when the AZ plant in Belgium got production problems the EU had to take the shortfall alone instead of it being distributed on all customers: the upshot of this is that UK has simply gotten more vaccines than the EU. The second problem is largely national: some countries have been slow to use the severely reduced deliveries they have gotten (possibly because their schedules were screwed up by the shortfall). The national governments have been really happy to pretend the national programs are perfect and there aren't large stockpiles of unused vaccines in their countries, and to blame all the tardiness on the shortfall "from the EU".