I checked the 1870 Census for Isaac and Elizabeth Roberts. I did not find Isaac but I found an Elizabeth Roberts age 23 born in Illinois living with Elsa Honemaker. There were also two younger Roberts male listed there. There names were not readable. If this is your Elizabeth, these other two would have to be Isaacís brothers. It is possible that this is your Elizabeth and that Isaac is off somewhere looking for work or is involved in some kind of work that causes him to travel.
I find it hard to believe that Isaac would leave her so soon after marriage but itís possible. However, I suggest that you check the counties surrounding Brown County to make sure this couple did not move or already live in one of them. Itís also possible Isaac lived somewhere else and found Elizabeth in Brown county and married her there and then took her back home with him. There are some Powers families in Brown County in 1870.
Most of the LDS Family History Centers provide free access to the Censuses. You can check for the one nearest you at:
Although I located a marriage in your county in 1869 for Isaac B.
Roberts & Elizabeth Powers, I didn't find any likely census people to
request a lookup. My loss.
But I did want to tell you I enjoyed your site & how helpful it is to
researchers. Thanks so much.
Joy in MO