There is a myth that every second or third bottle of shampoo, you should change as your hair gets used to it and stops working. Its not true, the reality is you may have dry hair ( for example ) and you get some shampoo for dry hair, you use it and take care of your hair, get regular trims etc for the next few months and its starts getting better. Finally its feeling healthy and normal again since all your time and energy went into fixing the disaster it was.
Now the second or third bottle in, you don’t need that shampoo now because your hair is not dry and damaged, so you need to change. If you use the same shampoo its not bad but useless in terms of fixing anything as its not dry and damaged any more, makes sense right?!
Always just check if the products you have are working for you at the moment you get your hair done, maybe it needs changing or maybe its working just fine, filling up bathroom cupboards with products is not what we do, nor should you!