In my opinion, 1.5 hours is not enough to charge the headphones, charge them overnight.
If you have the same problem, return the headphones. I have them too but never had a problem.
I bought the headphones today. The battery was empty. I listened to them for at least 1.5 hours and then tried them out. After five minutes, the left ear already got the message that the battery was almost empty. I plugged them back into the powerbank (has enough charge) and noticed that only the right one was flashing red when charging. The left one has a red "stand light". 2 hours later, the right one no longer flashes (charged) and the left one continues to glow red. Again, after max. 5 minutes of use, the message that the battery is almost empty. Can anyone tell me what the problem is? I can't find anything about it on the internet. It's probably not the powerbank, I've changed the charging positions.