First, navigate to ~/Library/Scripts/Folder Action Scripts to determine if that folder action is still installed. If it is, then skip the next step and go to step three, below.
Second, if the folder action is not installed, go to DEVONthink > Install Add Ons … and install the Folder Actions
Third, open Folder Actions Setup (it is frequently in Applications > Utilities). Select the folder in the left hand pane, making sure the checkbox is checked. In the left hand pane you’ll see which folder actions are associated with that folder. Make sure the DEVONthink folder action you want to be assigned has been assigned and is checked. If not, click the plus-sign at the bottom of that pane and choose the correct action to assign.
Folder actions are not services. Were you expecting to see this in the Services menu?