It's actually a chemical reaction combined with a flaw in the E-11 blaster carried by stormtroopers, see: there are chemicals present in all non-cloned humanoids that are given off in perspiration. Stormtroopers get extremely nervous due to extreme consequences for messing things up (they heard what Vader did to Admiral Ozzel), and thus they perspire almost all the time. The chemicals in their perspiration soak through the stormtroopers' gloves and seeps into the tibanna gas chamber of their E-11 blasters, where it reacts with the tibanna gas and causes several variables to be created within the firing of charged tibanna gas particles that are commonly known as "blaster bolts". There are so many new variables in firing speed, direction, and power that the same shot can almost never be made twice in the presence of the perspiration chemicals, thus greatly reducing the accuracy of the blaster to the point where it is almost useless.
In blasters other than the E-11, there is no problem, because the tibanna gas chamber is more tightly sealed so the chemical cannot enter the chamber and cause reactions.
Of course, the Empire was too cheap to buy any other type of blaster for the stormtroopers than the E-11. If the E-11 had been more airtight, or the troopers less sweaty, the war would have easily been won by the Empire (Han and Luke would have been fried in their first encounter with stormtroopers, thus Luke would never live to blow up the Death Star, and it would probably blow up every Rebel-Alliance-affiliated planet in the galazy, crushing the rebellion permanently and extending the Empire's existence indefinitely).
