Water (rain or irrigation) and/or fertilizer and/or shade.

Miscanthus varieties tend to get a little floppier as they get older & bigger.
Zebra grass (Miscanthus sinensis 'Zebrinus') is often mistaken for Porcupine grass (Miscanthus sinensis 'Strictus') - same variegation but a much more upright form

Even my mature 'Strictus' is much looser with age. If we have rainy weather when the grass is first flowering it falls right open.

Solution??
Next year divide your grasses - smaller clumps = tighter clumps
or
Based on advice from this site a couple of years ago; loosely tie your grasses up long before they droop. This works really well. And, if you do it early enough you don't see the twine.

thanks for the help. I think I'll be moving it next year. I had no idea it did this. Any suggestions on what to put in it's place? something similar, no bigger than the zebra grass? It gets part shade I'd say. Not much morning sun. thanks again!