I had this issue and it took very long to fix. In my case there were two culprits: opacity and drop-shadow. Setting opacity breaks the blur effect so you should (just like in this answer) use rgba(r, g, b, *opacity here*). Second one is drop-shadow. If the element I wanted to blur also had drop-shadow it adds shadow but blur disappears. I hope this helps.
Does your blurred element use opacity? Maybe eliminate and swap out for a color that uses transparency instead?