You have found your ancestor in the 1820 and 1830 censuses, but he cannot be located in the 1840 census. What can you do? There are several approaches, but one idea is to locate his 1820 and 1830 neighbors in the 1840 census and see if your ancestor is nearby with his name woefully misspelled or written in a barely legible fashion. It is possible that your ancestor moved out of state; locating those 1820 and 1830 neighbors in that “new” location may allow you to find your ancestor living there among them.
Of course, it is always possible that your ancestor is dead in 1840 and not enumerated at all. And there is always the chance that if he is living with one of his grown children in 1840 that the grown child is listed as the head of the household. In this case, the ancestor is there, but just one of the “tic” marks for an older family member.