What kind of shui xian is it? Wuyi shuixian? High fire or low fire? Old bush or standard?
I haven't noticed a vegetal flavor exactly in most shuixian that I've tried, but shuixian is not always the most durable tea. It's possible that the tea just isn't that durable. While there are some shuixian that can go quite a while, I do think the initial taste can fade fast with many (especially non-old-bush shuixian). While I understand the urge to push a tea for as many rounds as it can go, I do think there are times where it's better to stop while you're ahead (i.e., when the tea is at its peak). This is very common in orthodox Chaozhou gongfucha. You can still brew the tea in a larger pot later for casual drinking if you don't want to waste leaf.
See this recent post for a bit more about that.viewtopic.php?p=234916#p234916
Some other random thoughts:
1) The tea may just not be that great,
2) You may simply prefer other teas
3) You may just need to appreciate the way the tea changes, rather than expect it to keep the same taste you get in the first few infusions.