If you're wondering why you can't develop the boss blueprints for the weapons, aux, etc. While you are for sure took the blueprint, and can craft the armors just fine...

It turns out you need to have access to the secret factory!!



And the way to access this factory is through the secret ice cream shop



When you talk to the salesgirl in front of the shop, she say you just need to eat enough ice cream -- i got gold stamp card, and i still can't access it, so that is definitely not the cause!!

Now the big question is how to access it, for some people it is unlocked and some doesn't, at first i thought this is a bug, as this is mentioned a lot in the bug report thread, but no developer responded to it, while responding to other bug, so i was curious..

I tried eating all the ice cream variation (16 types of top) and i still don't unlock the factory
Until, when i was exploring the map to fight the world bosses, and mine some ores, when i went back to the base, it suddenly unlocked.

• buy some ice cream, i got gold card (some people report they unlock the shop at silver grade)
  
• progress story mission until you unlock the legendary recipes -- i finished all the story mission, but i highly doubt this is the requirement
  
• Mine some ores!! << very likely the the cause. Mine in your own session!! since i already mined several ores in another person session, but the factory is not opened, until i was mining in my own session.
  
• some people report they need to develop some weapon/armor until they open the shop, try finishing sidequest Exciting arms development! that create a knuckle (credits to @Blacky Watchy)
  
  What you don't need to do:
  
• ranking up the colloseum -- i unlocked it and didn't bother with it yet, so i ranked 2 (from the bottom)
  
• i don't play the cards minigame
  
• i barely do side mission, i did play several on other people session and got redacted reward (i don't think it counts since he got the music NPC, and i don't in my own session) i don't do any on my own session yet since it's a chore to redo it
  
• beat all world bosses (as you can see i'm still missing a lot of cores)

Hope this helps for those having a horrible time with the secret shop, Please do drop a comment and like if you find this guide helpful, and would love if you also share your findings too if i missed anything, to help others in the same cause. cheers!