Manamune doesn't charge with some Jayce abilities
It's passive says: Unique – Mana Charge: Grants +4 maximum mana for each basic attack, spell cast, and mana expenditure (occurs up to 2 times every 8 seconds). Transforms into Muramana item.png Muramana at +750 mana. But it doesn't seem to work with Jayce's Hyper Charge (cannon W), or Acceleration Gate (cannon E), they both cost mana so i think it's a bug to not give stacks. Jayce is one of maybe 5 champs that build Manamune and it's stacking very slow now and with this bugs it takes even longer until it's fully stacked.