If the straps become loose immediately upon fastening the shel yad, they should be refastened without a new brocha. If, however, the shel yad or shel rosh became loose after the shel rosh was put on, a new brocha is necessary when refastening it. If the loosening took place during davening, a new brocha should not be said. Since it is understood that a person wears tefillin throughout davening, it is as if one was mindful at the time of the first brocha that it should cover all replacements during davening.
[סעיף יב וס"ק נא-נב; וראה ביה"ל ד"ה אינו]