As I see it there are two things going on: not seeing the results you want from the activity you are doing, and not understanding why strength is not linear.
I'm going to attack these in reverse order. Take a look at the chart below:
Depending on the training stress, your body is too fatigued to demonstrate any kind of strength. It needs to recover, or rebuild itself to handle the new demands you place on it. If you understand the way your body reacts, you can time the next stress while you are at the top of the super compensation curve. Super compensation means that you are stronger than you used to be. When you have the grip of a 10 year old, it is likely because your hands are fatigued. If you keep adding stress, you will eventually get weaker because your body can't recover.
In short, you are correct, it's not linear.
Think about the type of grip you want. Pinching is not the same as holding. Rotation is also not the same thing as holding. If you want to crush someone's hand, you need to hold really heavy things. My suggestion is that you don't train your grip more than once a week, and ideally after you train deadlifts.
- Static holds: load up the bar as heavy as you can hold, and hold it as long as you can. Stay at the same weight until you can hold it for at least 30 seconds. Increase the weight and repeat.
- High rep 1 arm dumbbell rows. 350 training is very effective: attempt to get at least 50 reps in no more than 3 sets. When you can do that, increase weight and repeat.
You can do static holds on the last rep of deadlifts. If you aren't doing deadlifts, it might be a good time to start.