Why is my prickly pear cactus drooping?

There was a time when it stretched its leaves upright, but now it hasn't done that for weeks. I only ever gave it water when the substrate was dry again. Maybe it's missing nutrients. I don't have any direct cactus fertilizer, but I do have normal plant fertilizer. I've also read that if the new leaves are too thin, it's not getting enough light, but that can't be right. It's in a lot of light, and often in direct sunlight on the balcony.

I got this beauty from eBay, but the previous owners didn't take good care of it and it has a few defects… the substrate, for example, is really hard and impermeable… and it has some dark spots on the leaves. I suspect fungus or bacteria.

what do you think?