The answer to the question depends on the specific language contained in the Trust document.
The Trust may require all trustees to act jointly, or it may permit a majority or any one of the Trustees to act on behalf of the Trust.
Further, and depending upon the language contained in the Trust document, the Grantor has the ability to appoint and terminate Trustees as the Grantor sees fit.
If a Trustee is not fulfilling his or her duty as Trustee, then the Grantor may have the ability to terminate the non-performing Trustee.