Most likely, you have a big red flashing sign that says "Don't buy here, go away!" on each page of your site.
It's hard to be specific without seeing the site or knowing it's history.
Right off the bat I would have some friends/family make some test purchases to see if your shopping cart (or whatever you are using) actually works.
You may also want to consider your payment method. Are you Paypal only? It's a big turn-off if customers can't use a credit-card in the standard fashion.
Cheap prices don't sell a product (although it certainly helps). Do you have good pictures, description, etc?
Does the site look "unsavory"? Do you have obvious and clear contact information. A terms and policy page covering returns, etc? Are your terms bad? i.e.- no returns period
How old is the site? Are your visitors human or is your traffic nothing but bots?
etc., etc. etc.
There are many other factors. Do some searches on "sales conversions".
Depending on the product, it may be just the season - summer's always off for us too.
Yes it depends upon the product, specific target market and the market segments and customer behavior in certain situation. You should research and make changes or improvement accordingly.
How many visitors are you getting a day?
Assuming a conversion rate of 1% (just an example, could be much higher or lower depending on what you sell!) you need 100 to achieve a sale.
Prices can be too low. Your visitors might think your goods are low quality, you'll be gone tomorrow etc.
There's no reason to undercut your competitors by a huge margin. Try increasing your prices, so that you are just a little cheaper. Even if sales don't increase, you'll be making more money!
Markets are much sensitive and also its not homogenous, increasing price suddenly after offering 20% cheaper, I think will create bad effect to overall sales.
Sometimes if your prices are too cheap, people think you're a fraud. Customers have emailed me on this issue. I raised some of my prices because of it.
But right now it is slow for me. People are on vacation and others are starting to have to buy stuff for their kids for school, so they don't have much money left over to buy my widgets. I have noticed the usual (for me) dip in traffic for this time of year.
Remember your price should almost never be lower than your costs or higher than what most consumers consider (fair).You may set your price to achieve a target ROI.
'we are ranking in the 2nd or 3rd position for our targetted keywords'
Are those the keywords YOU want to be found on or the keywords your potential customers are searching on?It's no use to rank #2 or #3 for 'widgets' if you only sell green widgets with blue dots. It would be better to have a good ranking for 'green widgets with blue dots'. Wordtracker might be a good solution for you to determine what your best keyword is.
Off course, all suggestions already made are also true. Make sure your website can be trusted and be clear in contact information, refund policies, terms & conditions etc.