I had this both when it was brand new and after sending it in for the upgrade. When it was new, the ramp on the bottom of the barrel was where the friction was. After some grease and a few hundred rounds, it eased up.
After I got it back from the upgrade, it was worse than it was new. Luckily, I had an RX and another 320 and could do some comparisons of FCU, slide, striker etc. Turns out that the new X5 striker (even though they said it wouldn't need one) was binding as the slide came back into battery and cocked the striker. It smoothed out after a couple hundred rounds. If it hadn't, i was going to call customer service. If you can swap out the striker / slide with another to test, you might find the same thing. That striker still doesn't cock as smoothly as the others but it's fine under slide movement from full slide lock.