There's both a seperate dekane questline and some quests in dekane as part of the Reonan questline. Finishing the dekane questline doesn't mean you've done everything in Dekane from the Reonan line as well.
I'd check which the last Reonan quest is that your RM has done and then check the NPC that is supposed to give you the next one. You can use my questsheet for this: https://docs.google.com/spreadsheets/u/0/d/1mrdor5XtInhotJwI1gLHTRblsiIPmW9wqsRTvRpYvx4/htmlview#gid=988266989
Also check if you haven't declined the quest. If you decline a quest, you can still do it by talking to the npc that offers it, but the exclamation mark will be gone.