Please view the workaround below.
After completing a Stage in Invasions you may notice that the button is greyed out and shows as "Collected".
Fixing the problem
On the 28th March we experienced an issue where the amount of towers you needed to kill were lower than they should have been. This has been corrected to what it should have been in the first Invasion Event. If you collected rewards while the issue was occurring, you cannot earn them again.
Some stages that were previously completed now have higher requirements and you might have to defeat more towers to complete them again. If a stage was completed but you did not have a chance to collect it before the correction, it will be available again once your guild reaches the new requirement.