Because Harry is filled with a deep-seated self-loathing and thinks that if he ever tries to use Magic to make his life less terrible by even the smallest of degrees he'll either turn into a full-blown Supervillain instantly or the universe will reach down and squish him like a bug for daring to live any other way then in abject suffering just above the poverty line.

TLDR: Because Harry's an idiot.

This answer also covers other questions, such as:
"Why does Harry never again make any of those OP potions that solve half his problems"
"Why doesn't Harry ever find out about things like freaking Vampires until post-SF, even though he's had Bob for years"
"Why does Harry never look into healing Magic on any level, despite his suicidal lifestyle"
"Why doesn't Harry work on his water Magic, since that apparently counters a Wizards weakness to flowing water"

And so on and so on (Seriously, Harry is terrible at Wizarding)

Jim has addressed the first point: A wizard only has so much power to invest in their magical gizmos. At this point, Harry is rocking things like his staff, jacket, shield bracelet, and blasting rod (essential tools for amping up his wizardry) and so does not have the magical umph to spare.

Additionally, as OP as those potions were, they nearly got him killed, and are one-use only.

On the healing magic front: It is -incredibly- difficult. Listens To Wind has built his life around it, and even he cannot cure everything. Harry, who has less experience and less innate talent for slow, low watt magic, is unlikely to do anything meaningful with it.

Water Magic: Maybe you missed it, but Harry has been throwing around an awful lot of ice lately. Ice isn't, inherently, water magic, but Harry is still working within that realm.

For starters: Have a strong sense of plot. Know the events of your story.

After that? The fae like making deals, and they love information (and by extension, power) more than even humans. They gossip on a scale that gives them access to almost anything they want to know badly enough. Because of this, it's probably safe to assume they know the story almost as well as you do. This is a shortcut/aid in helping them plot further ahead, and risks making them seem omniscient, but with practice and a careful hand, can be done to great success.

As to speaking in riddles and not lying, remember that even more than Fae like knowing secrets, they -love- selling them. That's what makes that information powerful, the fact that others know it. They -want- the people they're talking to be wowed and enticed. Being cryptic to the point of obfuscating information or being frustrating to the players isn't the goal. As "Your Story" puts it, the Dark Powers are always willing to help. Make the first taste free, then hook them with the promise of making the cryptic comment clear.

In short, prep ahead of time. Not inherently to the point of rehearsing every line, but spend three or four hours really getting into what these creatures in specific know, and what it means to them. Maeve is going to speak cryptically in a very different way than say, Mother Summer.