Good ones are hard to find.

One syllable middle names. Bane of my existence.

Girls:

Belle
Blythe
Claire
Elle
Fay(e)
Gail
Grace
Jane
Jean
Jess
Jill
Joy
June
Kate
Kit
Lil
Lou
Mae
Maeve
Nell/Nelle
Paz
Pearl
Rose
Ruth
Tess

Boys:

Bob (potentially hick)
Cole
Drew
George
Gus
Hank
Jack
Jake
Job
Joel
Jude
Luke
Mark
Matt
Miles
Neil
Paul
Pete
Pierce
Rhys
Ron
Seth
Steve
Tim
Tod
Tom

Do you have to have a one syllable mn? Just thinking since it seems you are having trouble with them?

Girls:

Belle - Very sweet, kind of country just by itself to me. Prefer Isabelle or Annabelle or even Bella
Blythe - Love:)
Claire - ditto
Elle - Really like this, means 'she' in French so very feminine
Fay(e) - not really a fan of this but it's fine
Gail - really not a fan.
Grace - Love:)
Jane - ditto
Jean - ditto
Jess - too short, it's just not enough name for me. Jesse is a boys name so how about Jessica?
Jill - I like Jill:)
Joy - happy:)
June - okay, still old ladyish to me. Prefer April or May
Kate - Love:)
Kit - interesting, still not sure on this one
Lil - as in the tacky way of saying little? no. Try Lily or Lila or Lillian.
Lou - alright, nothing that jumps out at me though
Mae - really like this:) it's so cute
Maeve - not really a fan
Nell/Nelle - like this too
Paz - no
Pearl - Love:)
Rose - ditto
Ruth - like a lot:)
Tess - sweet, similar to Belle imo

Boys:

Bob (potentially hick) - not enough. Robert only.
Cole - like
Drew - ok, prefer Andrew
George - really like George:)
Gus - it's alright, prefer Angus or August
Hank - country. Henry is great though
Jack - everybody loves Jack but I prefer John
Jake - only nn for Jacob
Job - seems like a lot of name. Think book of Job
Joel - nice, kind of spunky lol
Jude - Love:) 'Hey Jude'
Luke - like Luke, not as much as Lucas or Luca though
Mark - really like this
Matt - only nn for Matthew
Miles - okay
Neil - great name:) Think Armstrong
Paul - okay
Pete - only nn for Peter
Pierce - Love:)
Rhys - like this
Ron - only nn for Ronald
Seth - okay
Steve - prefer as a nn for Steven
Tim - only nn for Timothy
Tod - Todd, I think is okay
Tom - only nn for Thomas

Good Luck:)

One-syllable middle names that I love:

Girls:
Anne
Bea
Blythe
Brynn
Claire
Eve
Fern
Fey
Hope
Jane
Jo
Kate
Kaye
Kit
Mae
Maeve
Nell
Paige
Pearl
Plum
Quinn
Ruth
Sage

Boys:
Blue
Bram
Brice
Clark
Clive
Frank
George
Gus
Hank
Ike
Jack
Jay
Joe
Max
Neil
Pete
Ray
Reed
Roy
Vance

Off the top of my head :
Boys' :
John
Jack
Jake
Tom
Tim
Max
Hal
Nate
Charles
Chuck
Luke
Seth
Brett
Ray
Bob
Giles
Chad
Lorne (lorn)
Frank
Fred
Fritz
Klaus (klows)
James
George.

Girls' :
Cate/Kate
Niamh/Neve
Maeve
Jane
Jean
Claire
Grace
Pearl
Jade
Brooke
Blythe
Rose
Hope
Faith
Joy
Beth
Anne/Ann
Belle
Elle
Lou
Mae
Dae
Rae
Rain
Ruth.

Good luck!

I'll just list the ones on my list, sorry for any and all repeats!!
Boys:
Art
Ash(e)
Bear
Birch
Bly
Bram
Claude
Clive
Clyde
Cove
Crane
Crow
Dune
Elm
Field
Fisk
Foy
Frost
Gray
Grove
Hayes
Jules
Kent
Klaus
Clove
Laird
Lake
Land
Light
Locke
March
Marsh
Moe
Moss
Noon
North
Oak
Pan
Peace
Poe
Quade
Quest
Quill
Reef
Reeve
Rook
Roux
Rune
Storm
Teague
Teal
Thane
Thorne
Thyme
Truth
Twain
Vine
Waite
Wales
West
Wolf
York

Girls:
Ari
Aud(e)
Bell
Bird
Blue
Blythe/Blithe
Cloud
Clyde (I consider this unisex)
Dawn
Day
Doe
Dove
Dream
Fawn
Fern
Free
Fife/Fyfe
Gale
Glade
Glory
Greer/Grier
Haze
Jules
June
Kite (for the adventurous... ;)
Lane
Lark
Leaf
Lynd
Lys
Maine
Maud(e)
Merle
Moon
Pax/Paz
Pearl
Plum
Rain
Roos
Scout
Sol
Snow
Song
Spring
Sun
Thaïs
True
Wish
Wren
Wyn/Wynn/Wynne

There's also King, Prince, Queen, Duke and such, but I don't like those, which I know is kind of weird 'cause I seem to approve of everything else XD

I think Wren for a girl and Claude for a boy are my favourites, but I also love Cove for a boy and Blithe (spelled like this since I think it looks lighter and better conveys the happy/carefree meaning) for a girl...
Best of luck!

More boys :
Pax
Shane
Zane
Zack.

And Fern for a girl. Oh, and I adore Kit!