Muscles need time to repair. Generally ~48 hours, but depends on how much damage was done, type of training, size of muscle, training status, etc. Analogy from Dorian Yates: if you scratch the palm of your hand with sandpaper pretty good and give it time to heal, callous will form. But if you scratch it again before it heals, you get a bloody hand. Recovering properly from exercise is about as important as the exercise itself. If it’s getting harder, it may be a sign you’re actually destroying more muscle than you’re building.