Gem Slots - Weapon Upgrade

[Otaner]

Guys, 

My brother upgraded his weapon to Baleful yesterday and all his gem slots were opened, I've upgraded my own weapon to Seraph a couple weeks ago and I still have only 3 gem slots opened!!

This is Random luck, or should have I done somethig that I've missed to do?

[Deadly Intent]

Completely random.  The next time you upgrade you might open 0 or all of the slots (you will not lose any),

[Otaner]

Thanks @Deadly Intent

So, everytime I upgrade stage 4 to stage 5 or whatever there is a random chance to unlock gem slots?

[Leyren]

When switching from the purple weapon to legendary, you get 3-6 gem slots. On every further weapon upgrade while having legendary, you have a chance to get the missing slots opened.

[Otaner]

thanks guys ^^

[Sir Bismarck]

I got no clue what the chance is but I've upgraded to stage 10 now and I've not unlocked more from the original 4 I got when I went to legendary