It's not so much that Eifert himself is doing anything wrong, it's more that Andy Dalton is just looking at other players, specifically A.J. Green. While Green started the season off slow, he's caught 17 balls for 309 yards and two scores over the past two weeks. Yes, 227 of those yards and both scores came in one game, but Dalton has targeted Green 23 times over the two-game span.
On the other side of it, Eifert has received just seven targets in the past two weeks, while catching three balls for 69 yards total. After hauling in 13 catches for 153 yards and three touchdowns in the first two weeks, he's dropped off completely.
Eifert will still have some great matchups and get looks from Dalton as the season goes on, but we may want to cool off on the Eifert hype for a few weeks, at least until he begins to get more targets.