Is IV Lopressor not working?

I recently had this thought that there is something wrong with IV Lopressor lately. Sure, it doesn't always bring down HR, convert people's rhythm, or decrease BP, but lately I haven't seen any of those happen even once. Tonight I have 2 patients both on q6h IV lopressor. I took their BP before administration, then after. No change in BP, or HR either. Have had a few SVTs which showed no effect, even after multiple doses. It just seems like it would work at least sometimes - at least I know it used to. Lately, however, nada. Anyone else noticing this, or is it just me?