Haskell, Aeson - how to debug instances?

I have a complex nested json that I am trying to parse with Aeson and Attoparsec into my custom types. Based on the information from the questions: Haskell, Aeson and JSON are parsed into a user type , Aeson: How to convert a value to a non-standard? and some data from the Internet.

When I use the following code, I get the value "Nothing" from the overlaid FromJSON instance, but the code goes through each instance for sure, I checked this by disabling some other insances. So, the main question : how to check the code in the examples and see how the data changes during execution in GHCi?

PS: I tried to set breakpoints and "trace", but they work only in the main and parseCfg-functions.

{-# LANGUAGE OverloadedStrings, FlexibleInstances #-}

-- high level data
data Cfg = Cfg { nm  :: CProperty,
             author :: CProperty,
             langs :: CValue,
             grops :: CListArr,
             projs :: CPropArr
           } deriving (Show)

...

instance FromJSON CProperty where
parseJSON _          = mzero
parseJSON (Object o) = CProperty <$> toCProperty o
  where
    toCProperty :: (HM.HashMap T.Text Value) -> J.Parser (T.Text, T.Text)
    toCProperty _  = error "unexpected property"
    toCProperty o' = do
      l <- return $ HM.toList o'
      k <- return $ fst $ head l
      v <- return $ snd $ head l
      v' <- parseJSON v
      return $ (k, v')

... lot of different instances

-- |this instance is specific for different files
-- based on common functions to work with most of nested json code
instance FromJSON Cfg where
  parseJSON _          = mzero
  parseJSON (Object o) = do
    nm     <- (parseJSON :: Value -> J.Parser CProperty) =<< (o .: T.pack "Name")
    autor  <- (parseJSON :: Value -> J.Parser CValue)    =<< (o .: T.pack "Author")
    langs  <- (parseJSON :: Value -> J.Parser CProperty) =<< (o .: T.pack "Languages")
    groups <- (parseJSON :: Value -> J.Parser CListArr)  =<< (o .: T.pack "Groups")
    projs  <- (parseJSON :: Value -> J.Parser CPropArr)  =<< (o .: T.pack "Projects")
    return $ Cfg nm author langs groups projs
------------------------------------------------------------------------------------

main :: IO ()
main = do:
  s <- L.readFile "/home/config.json"
  -- print $ show s
  let cfg =  parseCfg s
  print $ show $ cfg

parseCfg :: L.ByteString -> Maybe Cfg
parseCfg s = decode s