It's hard to measure XP gains from killing enemies with weapons, especially with exotic guns because of how much raw XP they need per bubble. Outside of bounties and mission reward xp bonuses (completing strikes, patrol beacons, etc), weapons will only get XP applied to them by killing enemies with that specific weapon. Unlike armor, ability kills (grenades, super, melee, etc.) do not give any xp to your equipped guns. Looking at the last two story missions you played on your warlock yesterday, you had 58 kills with bad juju, and a 74 with the fusion rifle. Since you had finished those story missions before, there was no bonus XP from completing the mission. That might not have been even 2000xp towards the fusion rifle in those two missions. In the context of legendary and exotic weapons, that isn't much and the upgrade bubbles wouldn't show a lot of visual change. I have had it happen a few times when looking at the details of a piece of equipment where the XP bubbles do not show any progress, like a quarter filled circle will be empty. Exiting the details of the item and then going back in usually refreshes to the right progression level. Also, the website and mobile app do not always refresh immediately.