Your dose was on a lowly side. I was taking 750mg tid (so 50% more), and even then I was on the strictes diet you can imagine. Here's an anecdotal evidence of how important the diet is: I was getting better slowly, and was on this strict diet. One day I was eating my own home made soup which had two baby carrots in it. I ate both of those baby carrots. We are talking a tablespoon of carrots if that, in a meal that had many other non-starchy things in it. It also had just a tad bit of onions to help give the soup some taste. The rest was brocolly, cauliflower, green beans and that was it. After the meal I had one of the worst attacks ever - those two baby carrots were all that was needed! And that was while taking Metronidazole.
If two baby carrots can set me back a few days, I can assure you that your medication given in insufficient amounts combined with bad food could not do much.
Also it is extremely important to be hydrated as that should prevent the parasites from forming into cysts.