Vladimar DoT not rending

my last game i had a vlad top lane and every time he would use his DoT (Not sure what button it is i dont play vlad) it wouldnt rending as a debuff or visible so having it on you would cause no reaction due to nothing rending till it expired for example i had 200hp and i also had heal pots i didnt use one on the disengage due to not know it was there another fight i had 700hp which he didnt kill me but instead of a tick i would get hit with the full damage it would deal as it expired