A dirty or clogged screen can cause the low water presser remove shower head and clean. If not it could be calcium deposits built up on shower head if so soak shower head in a cleaning solution for several hours if that dose not work buy a new shower head. (note some shower heads have a low flow re-stricter in some cases you can remove this and get more water pressure)
you have trash in your line take the shower head off and remove trash or possibly in water valve at shower
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 you have a new pressurised water system there is a possibility that the added water pressure could cause a leak.
My first thought is a leaky hot water line.
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.
More than likely a blockage, leak, or bad installation. If some other showers, toilets, and sinks are having problems with running water then it's likely a problem with the entire water system, usually bad parts or a leak.
Because it effects the water pressure of the shower. This causes the temperature of the water in the shower to also change.
There is a screen and/or a water saver in the shower head. The screen may be plugged with sediment from in the water.
Its a loss in water pressure to the shower.
May be build up in the valve itself or the shower head.
I will say you can take a look at aquasana shower water filter.
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.