Here are some things to try:
- Grease the smooth rods
- Check if the linear bearings run smoothly
- Check if the nozzle gets stuck on a print
- As RRUser said, adjust stepper motor current
- If you've over tightened the belt, it can happen that the motor is having a difficult time to get the x carriage in motion
- Check acceleration. Try to find out if a certain part is responsible for the missed steps. Some zigzag motions can cause the motor to stall,