Salesforce has a limit on how many records you can follow in Chatter. As of the Salesforce Winter '14 release, that limit is 500 records. Once you reach the limit, you will no longer be able to follow any additional records including TaskRay Tasks.
One of the things that you can do to mitigate the limits and continue following projects is unfollow archived TaskRay Projects. In TaskRay, under Tools and Settings, there is an option to auto-unfollow archived projects. Instructions for how to get to the Tools and Settings are found here.
You can select to mass unfollow all archived tasks and projects, as well as opt in to auto-unfollow tasks on archived projects going forward.
In addition, you might also want to check if you are auto-following records in Salesforce because that can contribute to reaching the Chatter limit significantly. You can read about auto-following Salesforce records here.
Another great way to check on what you are following is to go the Chatter app/tab from the Salesforce Classic toolbar, click on your name, and select the Overview sub-tab. In the right, column you will see a count and a list of records that you are following.
Once you are under the 500 limit, you will be able to resume following TaskRay Tasks.