Working lash serums for thicker lashes

Recently I've been noticing that the lashes on my right eye are falling off and last night I slept with makeup on and it got even worse. I notice a BIG difference between the lashes on my left and my right eye and I have no idea why the lashes on only one eye are falling off. Can somebody please recommend a lash serum that actually works? I am looking for something that would make my lashes rather thick than long and something that would work fast, because I am a bit insecure about it. I've tried castor oil but didn't see much of a difference. I'll be very thankful if somebody gives recommendations!