Hello. Issue sounds like a permissions restriction for the new users, at the project level.
The most common cause for this is often new users default to Viewer group and therefore cannot execute any change within a QC project. Now you might be quick to reply that you definitely put then in a specific user group within the project customization. That could be true, but one mistake many make is they do not remove the user from the Viewer group. While that group is not guaranteed to have any priority, having users in more than one group is asking for trouble.
HP strongly recommends users be members of one project group/role only. Many issues have been reported with having users in more than one group/role.
Check your new users within the project having the grayed out Run and ensure that the users are in one group only and the one group they are in does in fact have permissions within TestLab to actually run tests.
Log into a project that has the new users with an admin account.
Select from the top tool bar, the Tools tab and in the drop down select Customize.
Next select Project Users from left side options.
Find in the Project Users list on the right one of your new users and select their name.
Click to the right the Membership tab.
You will then see the ''Not Member of'' and ''Member of'' lists for the particular user.
Ensure the selected user is a member of one group only.
Since you are in Project Customization, click on Groups and Permissions from the left side pane.
Select the group your user is now a member of.
Click on the Permissions tab to the right and make sure this group role has the required permission to run tests. The TestLab tab will hold the permissions you would want to check.
Save your changes when done, and have the user log back into QC and check if they can run tests which they should be able to at this point.
Repeat above steps for your other newly added users.