I don't think you're doing anything wrong. as such, more that there's probably a combination of factors that might be contributing to the lack of sales.
The first one is likely to be the time of year, spring / summer are quite hot and thick, or longseleeve costumes probably don't sell well during these seasons.
Although we all know everyone has to start somewhere, and many people have no hesitation in purchasing from a new seller, others will hesitate or look for a seller with more experience - I think you'd be able to combat that a little by making some changes to your description, and adding in some brief but reassuring terms of sale (one of the most asked questions of sellers is whether or not they will combine postage on multiple items to reduce costs, so having a quick statement in all listings to let people know if you do is a good start, the item location - Australia - would be another good selling point for local buyers, so a quick line that stock is in Australia may give some people a bit more confidence).
I would make the font larger, and include all the information about the item that a buyer can't tell just by looking at it (even the stuff that's in the item specifics, like material, whether it's lined etc (since babies' skin is generally quite sensitive) and maybe a basic measurement or two, since most kids will grow out of something like that rather quickly, I'd expect whoever buys it will want to make sure that the child will be able to wear it comfortably for a little while.
