Good morning!

I've had my ficus for a few months now. The initial pruning and repotting went fine, and the ficus showed a lot of new growth after the pruning, which was great! However, now many of the new leaves are getting brown spots on them, and I'm not sure of the cause. There are visibly flies inside of the indoor greenhouse/growlight enclosure I have for the tree, so I initially thought it may be pest-based and sprayed the tree with a 1% castile soap/water mixture. However, the problem has continued to worsen since then. Growth also seems to have dramatically slowed in the past two weeks. I don't feel like I'm overwatering it, but the enclosure it's in gets very humid, so maybe the soil is staying wetter than I think? Is there a reliable way to know precisely when the right time to water is? Is there anything about the brown spots that gives anyone a clue as to the real cause of the issue? Am I overworrying? Thank you in advance for all your help!