The first thing you need to do is rule out that there is a problem with the main water supply line in your home. Once you have done this, you can perform numerous checks to increase the water pressure in your sprinklers.
For example, if your sprinkler has a head with adjustable screws, try and turn the screws counterclockwise. This increases the water flow through the sprinkler head. However, this will only work for that individual head. If you have an entire system, this will not fix the entire issue.
You can also try removing some sprinkler heads around your yard and cap the plumbing line. Less active heads mean that the water pressure will be greater in the ones still in use.