Either the percentage of 20% chance to reverse engineer a standard item is the wrong number or someone has coded the random number generator for the engineering algorithm wrong.
I've noticed that it is almost impossible to reverse engineer certain items, in this particular case a barrel.
The item mentions the 20% chance, yet in 30 attempts I have gotten zero schematics. Now, I know that 20% chance does not mean that in 5 attempts you should be getting an assured schematic, because, yes, every attempt is a new one.
Random Number Generator is ok. You are just unlucky.