If it's only few tickets from a project but you can edit others, it's probably related to workflow property set on the workflow for the status the issue is in.
Adding to what @Florian Bonniec said, if it's all issues within one/multiple projects, you may not have the Edit Issues permission in the permission scheme associated with those projects.
You must be a registered user to add a comment. If you've already registered, sign in. Otherwise, register and sign in.
You must be a registered user to add a comment. If you've already registered, sign in. Otherwise, register and sign in.