May be build up in the valve itself or the shower head.
When the water pressure in your upstairs bathroom is getting low and you find a white sediment buildup in the aerator of the sink, it is the most likely cause of low pressure. You can also find the buildup in other fixtures including the shower valve and cartridges. Cleaning the sediment from the parts helps restore pressure.
If the water pressure is fine in the rest of your house. The supply pipes are either too small for how many fixtures they serve/the shower's distance from the main. However, more than likely you have galvanized pipes that are corroded and they need to be replaced with copper or pex.
you have trash in your line take the shower head off and remove trash or possibly in water valve at shower
Is more water comming out your tub spout? if so change spout
american
pressure and humidity
If the shower curtain is in bad shape then it need to get replaced ASAP. Shower curtains are very cheap. If people go somewhere like a dollar store they can get them for one dollar.
This does vary slightly according to the shower, but for a shower without a pump, using a gravity feed, a water pressure of at least 0.1 bar is recommended. If your water pressure is low you will need to install a pumped shower.
Yes
The flow of water from the shower creates a low-pressure area inside the shower stall, causing the shower curtain to be drawn inward due to the difference in pressure between the inside and outside of the shower. This phenomenon is called the Bernoulli principle, where higher speed of a fluid (in this case, the water spray) results in lower pressure.
The simplest and lowest cost solution is to replace the shower head with one designed to increase the pressure.
The downward movement of the water in the shower creates downward movement in the air inside the shower area. Air flow causes low pressure and so the air pressure from outside the shower area is able to push the curtain inwards.