Too much water, too much fertiliser. Especially the latter, I suspect. Way, way too much of that. Every fortnight is too much for the majority of houseplants, and that goes double for succulents, because they are naturally slow-growing, and so don't need much feed.
Succulents are almost impossible to kill with neglect. I water mine maybe once every couple months. Haven't fed them or repotted them in years. I wouldn't say they are thriving, but they are certainly alive and well.
I wouldn't suggest you do exactly as I have been doing; not if you want them to actually thrive. But it does illustrate the point that, with succulents, if in doubt, don't give them much water or feed.
I would suggest watering them monthly (unless it gets really hot, then you could probably step it up to 3 weekly), and feeding every 3 months.