Windows update issue

I have 600 machines where windows 7 standard is installed. All machines are getting updates from wsus. All of sudden 30 machines are skipping to detect monthly roll up, security only update and IE11 updates. But these machines successfully detected.net updates and installing it successfully.

I am not sure if this strange behaviour. How comes these machines not willing to take the monthly roll up and ie patches. Wsus report says not applicable. Mabel install days not applicable as well. Preview patch is not installed