FPO stands for a follow-on-public offering. Details of how an FPO work are discussed in this wikipedia article.

Some reasons why FPOs are potentially not very rewarding on their listing day:

1. If the FPO is done at a steep discount, many large investors would still hold the shares as they won't be getting 100% allotment in the FPO.
2. However, there will be a considerable sell off if the offered price on the FPO is much lesser than the current market price.
3. There will be an additional sell off and corresponding drop in price from ppl cashing in on the listing gains.

Quoted from a response on a blog:
"The best price for an FPO would be at a slight discount to the current market price."

Comparison of historical data from some sites has shown that though IPOs show tremendous listing gains, the FPO pricing leads to modest to no gains. This matches with all of our observations above.

Another factor in this pricing, is the historical price of the issue. If there has been a run up in the price of the issue before the FPO, the discount in the FPO really does not hold. However, if there has not been any run up, it might be worth considering the FPO as being at a slight discount.

References:
1. http://www.financialexpress.com/news/fpo-pricinga-cause-for-concern/76001/0
2. http://www.e-investing.in/showthread.php?t=3180