In modern slot machines, the design of the virtual reel determines the chance odds of getting a particular symbol or set of symbols.  Virtual reels have more stops than actual reels do so several of the stops on the virtual wheel are tied to one stop on an actual slot machine reel.  In other words, the odds of landing on a particular symbol on the actual reel are based on how many virtual stops are tied to the actual stop.

In the normal weighted type of slot machine, the most upper jackpot stop (the one with symbol of the highest-paying jackpot) on each reel corresponds to only one virtual stop. Consequently, there is a 1 in 64 chance of getting the jackpot image on one reel. The chances change, however, to 1 in 643 or 22,144 of getting the jackpot image of all the reels if all of the several reels are put together in the same way.  For those machines that have a larger jackpot, there may be many more stops on the virtual reel. As a result, the odds of winning that particular jackpot are notably decreased.

It’s also possible that the losing blank stops seen above and below the jackpot symbols can correspond to more virtual images than other images.  This makes it more likely for a player to get a blank stop that is right next to the winning stop.  This gives the player the impression that she has “just missed” the jackpot, which, in turn, encourages the player to continue gambling, when in fact the proximity of the actual stops has no bearing.
The program on each machine has been carefully designed and tested to realize a particular payback percentage. The payback percentage is based on the percentage of the money that has been placed into the machine and that eventually gets paid out to the player.  For example, a payback percentage of 90, would give 90 percent of all the money placed into the slot away as winnings and 10 percent to the casino. Since all payback percentages are less than 100, in time, the casino wins out.
Most gambling jurisdictions legally require that the payback percentages surpass a certain level (usually around 75 percent).  Most casinos go beyond this minimum and offer a payback between 90 – 97 percent.  They do this because they know that if their slot machines are too tight, customers will take their business to other casinos.  The computer chip inside the slot machine contains the program linking the payout odds to a particular machine.  Unless the chip is replaced, the casino can rarely alter or change the odds on the machine.  Contrary to rumor, there is no readily available method that would allow a casino to instantly ‘tighten up” a machine.
Nor do machines ‘loosen up’ themselves either.  This means that playing longer doesn’t make the machine more likely to pay out.  Your chances of hitting the jackpot are exactly the same every single time you pull the handle, since the computer always draws up new random numbers. At least with the standard system, there is no such thing as a machine just about ‘ready to pay’.