We have a very strange problem. At the moment i was setting all the first and second admins of our site collections inside "central admin" - "site collection administrators" to the same account. For 27 of the site collections this is no problem, but now i have 2 site collections i cant resolve the account i need inside people picker. When i go to other places i always find that account and also in those 2 site collections i can resolve the account. So only for 2 of our site collections below "central admin" - "site collection administrators" i can't. Anyone has a idea what this can be?

Thanx. It was a problem with the peoplepicker. The sites i wanted to add the admin account did had the right peoplepicker, the others not and where still on authenticated users. So we need to configure our people picker right first.