I was playing Sion
top and got really low. As I made my escape with my ult backwards I got ignited 
but I proceeded to run towards my tier 2 turret because i knew it would give me a shield. The turret gave me somehow no shield, and I died. I stopped my ult when I reached the second turret, and there's no way I was out of reach.
When I ran back to my lane I got a shield from the turret as usual. How come it be?
There is no bug here, it's working as intended, you just didn't understand how it's SUPPOSED to work. The short explanation is: the shield doesn't "grow" or increase on people that are considered "in combat". This is why if you DO have the shield, and you take damage from something, the shield doesn't instantly refill.
It worked that way initially, but they changed it so that people couldn't just run into it and save themselves from Ignite, or other DoT Abilities. It's meant to be used as a shield while camping under the turret. Not a free escape card from DoT.
