There are now 2 quests in Skyrim I can't do. Mjoll won't give me her quest, no matter what I do. I've done almost every quest in Riften (only ones I haven't done, atleast according to the strategy guide, are the Temple of Mara quests and get the alchemy ingredients for Igun Black-Brair), emptied my misc quest log of almost all quests (I have talk to Dagaine, a couple of bounty quests, kill a dragon, talk to the Companions about work, and join either the Stormcloaks or the Imperials). No luck whatsoever. I'm Thane of all cities but Windhelm (including Riften). I told her I hate the Thieves' Guild (but joined it anyway, I don't think that's a problem though, I reloaded a MUCH earlier save and tried getting it before joining the Thieves' Guild, but no luck there). I tried wearing an Amulet of Mara to get her quest, still no luck. I even tried using the console (setstage FreeformRiften09 10, and giving myself Grimsever, hoping she'd take it back), and had no luck with that. It seems like if I hadn't done a lot of research trying to find out WHY I can't get that quest, I wouldn't have known she even GIVES a quest. It's getting a bit irritating to say the least.
A second quest I can't start is The Wolf Queen Awakens (the courier has never shown up to deliver the letter starting it). It maybe related to not being able to do Mjoll's quest.
A third problem is I once killed a dragon at Bonestrewn Crest, and it hasn't respawned, and my dragon bounty quest is for that one dragon that's not there. I've killed other dragons at altars (Ancient's Ascent and Shearpoint comes to mind) a couple of times since getting the quest, but that one refuses to respawn.
The only idea I have as to why I can't start (or finish) those quest is that I cheated my way up to level 81 (like I do in all the games I play. I utterly DISPISE being low level.), and added quite a few perks that I couldn't get because of the limited number of them.