Sue, my WWII reference has limited info on 637th TD Bn. It says they departed the US from San Francisco 26 May 1942; arrived Fiji Is. 28 June 1942; arrived Espiritu Santo 8 July 1944; arrived Bougainville 19 Oct 1944; arrived Philippines 9 Jan 1944. The same reference gives them campaign credit for Luzon (which campaign had inclusive dates of 15 Dec 1944 - 4 July 1945) but no campaign credit for Northern Solomans (which campaign had inclusive dates of 22 Feb 1943 - 21 Nov 1944).
Bouganville is in the Solomans, and their listed date of arrival (19 Oct 44) is within the dates of the Northern Soloman campaign. I'm not well enough informed to be able to tell you why they would not have received campaign credit for N. Solomans, also. Maybe someone with more knowledge can help.