I found Isaac Plumley on the 1850 Census-MA/Hampden Co/Ludlow/pg.322/Roll319 in the household of Elijah Plumley- 47/m/farmer/MA;Clarissa-43/f/MA; Lucy-22/f/MA; ISAAC-16/m/farmer/MA; Angeline B.(change of spelling and, therefore, (?) not children of Elijah and Clarissa) "Plumly"-5/f/MA; Jane E. "Plumly"-1/f/MA.
Then in the "Biographical Sketches of Hampden Co., MA"(Heritage Quest.com) I found that Elijah Plumley, b. Monson, MA, 11 Ap 1803, d. 1878, son of Isaac & Tamson Barrows Plumley, married Clarissa Hayden, b. Belchertown, MA, daughter of Moses and Mary Buxton Hayden and died 1888. Their children were Lucy, ISSAC, and Jane E. Plumley (wife of George D. Green). (same children as listed on the 1850 census)
1860 Census, MA/Hampden Co/Ludlow/Palmer Roll 504/pg. 953/line 16-Isaac H. Plumley-26/m/farmer/MA; Charlotte-26/f/MA; Chas. 5/m/MA.
So, it looks like to me that Issac was the son of Elijah Plumley and Clarissa Hayden Plumley and was first married to Charlotte and then to Mattie, 25 years his junior, so he probally died after he was 68 (the 1900 census and why you can"t find them) and she went home to Florida.