Q.grass falling over
I planted 4 small pots of grass three weeks ago three are ok, but the fourth is falling. Looks alive but won’t stand up. Used different brands of potting soil for two of them and a mix of two year old potting soil for the one that is falling.

The weight of the grass will naturally fall if it becomes to tall. You can trim the grass to a more manageable height.
Here are some links for you.
https://www.gardeningknowhow.com/houseplants/grass/grow-a-grass-houseplant-growing-grass-indoors.htm
https://www.gardeningknowhow.com/houseplants/grass/types-of-indoor-grass-plants.htm