An IPO is a mechanism for a company (and its investors and other stakeholders) to sell shares in exchange for money.
The investment bank that facilitates the IPO has the job of "marketing" those shares (and doing the administrative stuff around the IPO).
The goal in selling something is to get the best possible price. Therefore, a stock price that goes up at the date of the IPO and then declines below the initial price is the GOAL. It means the bank has done its job well. It got the maximum amount of money out of buyers.
If the price stood level, it would mean the bank had left money on the table. If the price went up immediately and then STOOD THERE, it would mean the bank severely underpriced the IPO. Thats the worst case scenario. If the price dips immediately and nobody buys, it just means the IPO was overpriced.
This also means that buying at IPO means that you're getting screwed unless you want to hold longterm. You're always paying a premium, unless you believe that the investment bank made a mistake. And you obviously have more data to make that bet than goldman sachs does. Obviously.
As a smalltime investor, for whom there is essentially unlimited liquidity in the market, just don't buy IPOs. And don't get excited over IPOs dipping below opening price. It just means you don't understand the mechanism.