The 'problem' (or feature, depending on your timing) with flightfox, darjeelin and others is that you're relying mostly on part-time experts looking for flights for you. So if you pick the wrong time, it may take a while to get someone on to look at it for you.
Also, and I found this especially with flightfox when I was an expert there, complex flights tend to get less attention from the experts unless there's some more money involved.
It comes down to cost-benefit. If you're an expert and you have an hour to search some flights, you're going to look at the ones you think you can do well and get cash for. So for me, when I saw a south Pacific one, I'd usually target it as I know a few little lesser-known sites and airlines. I also know tricks in South America. Europe the competition is just crazy, so I'd often avoid those - and there were too many little weird details - some people would be fine with a RyanAir airport (eg Vienna's one is in a different country - Bratislava, Slovakia!), while others wouldn't be, some would be ok with trains, etc etc, and then you had to factor in trains or boats as an alternative - it was messy.
So my suggestions from my time in it, as an expert wanting to get a win:
- be clear. Describe everything you need. It's so, so frustrating when someone doesn't mention baggage, you find a great flight, then they respond 'oh I needed baggage, plus space for my skis'. Where's that in your request??
- be simple, if you can. Sometimes there were ones where they'd list 5 frequent flyer programs that they were on, and how they had 3 children, aged 1.5, 7, and 13. Good grief. Some sites don't cater for searching for that very easily. Sure, if you have a family you want a deal for it, but it's an extra layer.
- be flexible. Wanting a flight between 7 and 8am on a Tuesday? That's cool, but it's harder to find a flight that beats the other price that someone's already posted, I'm not going to bother trying to beat it. If it's flexible I can look at weird airlines or other tricks for you.
- time or day or week? Nope, not that I know of - you could ask them I suppose, but assuming a lot of their experts are US based (may not be true - I'm not), perhaps evenings US. But a lot of the time it's just a bit of luck.
- finally, if you can beat the price, why not join? ;)
Also, consider posting in the chat sometimes. Occasionally one of us will be in a price-hunting mood, and happy to help!