When do I need type annotations?

Consider these functions

{-# LANGUAGE TypeFamilies #-}

tryMe :: Maybe Int -> Int -> Int
tryMe (Just a) b = a
tryMe Nothing b  = b

class Test a where
    type TT a
    doIt :: TT a -> a -> a

instance Test Int where
    type TT Int = Maybe Int
    doIt (Just a) b  = a
    doIt (Nothing) b = b

This works

main = putStrLn $ show $ tryMe (Just 2) 25

This doesn't

main = putStrLn $ show $ doIt (Just 2) 25
{- 
  • Couldn't match expected type ‘TT a0’ with actual type ‘Maybe a1’
  The type variables ‘a0’, ‘a1’ are ambiguous
-}

But then, if I specify the type for the second argument it does work

main = putStrLn $ show $ doIt (Just 2) 25::Int

The type signature for both functions seem to be the same. Why do I need to annotate the second parameter for the type class function? Also, if I annotate only the first parameter to Maybe Int it still doesn't work. Why?