Raising kids is the hardest thing I’ve ever done. By a nautical mile. Our middle child gives me the most grief at the moment, I often feel like I’m just not getting through to her. For example, she says “por nada” (you’re welcome) when she should to say “gracias” (thank you), like she’ll say “Por nada para la rica comida papi!”. Even though we’ve pointed this out to her countless times, she keeps making the same mistake. She’s slow in learning to speak, even at 4 years old, I don’t know to what extent that factors into it.
Of course there’s the normal kid stuff that drives parents crazy. No matter how many, many times we’ve asked our 2 oldest to be quiet when their baby brother is asleep, it never happens. Clean up your toys at night? Hang up your coat when you come in the house? Not happening.
We’re staying at my parents’ house for a while and the other day my oldest asked me about the barometer on the wall. I’m not sure she’d ever seen one before so I explained to her it’s an instrument that helps us predict the weather, that the atmospheric pressure changes as weather systems move and the barometer helps us predict if it’s likely to be sunny or cloudy and rainy.
My middle daughter was in the room but she wasn’t paying any particular attention to the conversation. Then 2 days later I happened to pick up my middle daughter as I walked past the barometer. She looked over at the barometer and casually goes:
“Hey, there’s rain in the forecast.”
Go figure.
Life in Peru 
Even when they are not listening, they are listening.
Your middle one is not slow, more likely bored. She shows very high intelligence at things that interest her. Watch out, she will challenge every teacher she encounters and may be falsely labeled as a slow learner or problem child. Keep your emphasis on her intellectual interests and capabilities and she’ll amaze you with what she understands.