Why is the water pressure in my shower getting steadily worse?

We have removed the shower heads (it’s a double shower) and cleaned them, so it’s not that anything is clogged. We are careful not to run the sprinklers or dishwasher or laundry at the same time as the shower, so it’s not that. One of the shower heads for a couple months had great pressure, while the other one started to get lighter, but now they are both losing pressure. When we first moved into the house they both had great water pressure. Now they both seem to be steadily getting worse. What could be causing this? Do we need to call a plumber or could we fix it ourselves?