Honestly ive had the same issue with farmhands as not all the girls you meet would like to join. Its really just RNG with the girls you meet in the game, because i meet a cute girl i wanted to have as a farmhand but she refuse to be interested.
At first i thought it was just because of their personality, but then i find someone who has the same personality and agreed to join, so its really just RNG when it comes to characters that spawn in the game.
But its kinda bad if you only see girls who refuse to be farmhand no matter what, leading to that you cant experience the farm feature.