The routine you settled on worked beautifully for a while and then stopped, and you changed nothing. Usually the product hasn't gone bad. Something around it has changed.

The first and most common is buildup. Shampoos remove different things at different rates, and the same bottle can handle one thing beautifully for years and never quite handle another. When something settles on your hair faster than it rinses away, you don't notice it happening — you notice a month later, when your hair sits heavier and holds a shape less well. Changing shampoo, or alternating two, is usually what fixes it.

The second is that you changed a different product from the one you're blaming. A routine works as a set. Add a mask and the shampoo that used to cope stops quite coping. Switch to a gentler shampoo and your conditioner seems to have forgotten its job, though it is the same conditioner. Before you replace the thing that stopped working, remember what you replaced just before it.

The third is that your hair itself changed. After bleaching, colouring or chemical straightening the surface of the hair is different, and what used to hold onto it now slides off. Quantity doesn't help — the same product, more thickly applied, gets you nowhere. Products built for damaged and coloured hair hold onto that surface.

The fourth is that your water changed. After a move this is the first thing to suspect. Hard water leaves a film on hair, keeps shampoo from lathering and working at full strength, and clings more readily to hair already worn by colouring.

The fifth is that the air changed. A style holds by way of bonds that water breaks. The damper it is around you, the more of the work your styling product has to carry on its own, and at some point there isn't enough of it. Same product, same application, different weather.

If none of those fit, step back and remember when exactly things were good, and what happened immediately after. The answer is almost always there rather than on a shop shelf.