You're describing lateral raises, and because the lever (your arm from shoulder to hand) is so long, you are more mechanically disadvantaged vs when you are doing curls. So it's normal that you can'f lift as much.
Different exercises can have different training frequencies. You can train arms 3 times per week, but deadlifts for example, work huge muscles and are much more exhausting, so once per week is typically enough for those.
Rep ranges for hypertrophy are typically anywhere between 8 and 30 (or more). So stick with 15lbs for the lateral raises and increase your number of reps over time. Once you hit 20 for example, then try increasing the weight and dropping the reps back down to 8, and work your way back up, repeat
The fitness wiki is a great resource, read up a bit and hop on one of the routines recommended there. It will answer a ton of your questions and also give you incredible results compared to just winging it