How did you apply the shampoo?
How long did you water after the shampoo?

Would you characterize the hard soil as either a low spot or a high spot relative to the rest of the yard? or about the same?

How long do you normally water?
What kind of sprinkler system do you have?

Have you checked the sprinklers in the hard spots with cans to be sure those areas are getting the same amount of water as the greener areas?

Hard soil and compacted soil are different things. Compacted soil has had the air mechanically driven out by first soaking it and then repeatedly sticking a plunger of some sort into it. Adobe bricks are made this way. In nature this typically happens around a pond where cattle or other livestock drink. It can also happen in a pasture if a farmer forgets to move his livestock out during a heavy rain. In a lawn it can happen if the yard gets heavy traffic during or immediately following a soaking rainstorm. Heavy traffic might be a party or a football game. Hard soil is caused by a lack of beneficial fungi which normally soften the soil. That can be caused either by prolonged drought or by short term flooding. in one the moisture is cut of for weeks, and in the other air is cut off for just a few days.

If you have seriously given up on shampoo and watering it back to health, then you could apply a thin layer of compost. Apply no more than 1 cubic yard per 1,000 square feet. Even bermuda can be smothered by too much compost.