HI HeartsOfPalm, welcome to ATP! First off I'm not sure you have a Colocasia there. The upright habit looks like it may be an Alocasia but it's hard to tell, at least for me, without closeups. LariAnn can probably tell from these pics. Either way sandy soils have to be amended with
lots of organic matter. Lots of mulch or compost on top are helpful as well. Here in z6a Massachusetts full sun is fine but in z9 much more shade is a must
or consistent moisture. Where are you? If your climate is arid you have to water much more frequently. Lower leaves that die-off are normal. By allowing them to dry up the plant reabsorbs nutrients. I sometimes cut them off early just to keep them tidy. Water soluble fertilizers, if that's what you're using, should be added much more frequently when the plant is in active growth.
If you can't move the plant, recommended, dig a trench around it and fill with compost. That may help but I think it will be a loosing battle in the long run.
Edit: I wanted to add that if provided with the proper culture, some enjoy full sun.
