Could be a few things.

Do you have them planted in a moist area where they will thrive? (Unless it's something like Blue fescue that likes dry soil.) Grasses will be smaller in size in poor conditions. Or they might need divided. You'll need a great big shovel & an axe I'm guessing : )

Jen

When plants in my garden don't thrive or seem to decline, it's usually because they are improperly sited. That is, plants that like shade in locations that are too sunny. Or plants that need very good drainage in locations that are too wet, etc.

What varieties of grasses do you have in garden? Most OGs prefer a lot of sunlight and good drainage. It could be your locations are too shady or too wet.

When plants don't thrive for me, usually I try moving them to a new location or transplant them to a container with good potting soil. Often these plants thrive after moving but other times they never seem to do well and gradually shrivel away. In addition, some perennials are just slow to become established but eventually thrive.