It could be many different reason - pls. post a photo of it. Couple of different views would help if possible; also let us know what is your general location and how you take care of it. Is it indoors? - outdoors? - what is it potted in (soil & container)?...

It is potted with 4 other small cacti, stone and sand decorate the top of the soil and it is kept indoors. I water it as instructed by the label but have failed to provide it with tomato feed as I have been unable to obtain any. Thanks!

Has it been in the same spot and the pot for a while? What is the soil like you potted them in? Succulents need very well draining potting medium to be in.

Looking at the 2nd photo, it seems to be dark at the base - possibly rotted and that's why it is leaning. Is it also loose in the soil? I would take it out and check the bottom of the stem to see if it is black/dark, dry or slimey, too soft, if there are any roots and conditions of them.

Stone on top is ok (some ppl do not like anything on top of soil), but sand not so much. It is too fine and clogs up any space available to breathe. Can't tell anything about the soil your succulents are potted in. If it is just bagged soil (top soil, potting soil, garden soil, cacti & succulent soil...) it may be too water retaining. If that's the case, the roots could be sitting in wet soil for too long, can't breathe and eventually start rotting .If there is lots of peat that became too dry, the water will just 'slide' over it and plant doesn't get any.

Succulents need lots of good light and sometimes they stretch or lean toward it if they are not getting enough. I would suspect, if that was the case, your other plants doing similar thing.

Your other plants - from whatever is visible in the photos - look ok. Did this plant have any roots when being potted up? BTW, tomato feed/fertilizer isn't needed, and lack of it did not cause this problem in my opinion.