One of the most common areas I get asked about when coaching is backspin. Many beginners struggle to play backspin shots and even good players will strike the cue ball far harder than required to impart spin.
The main faults I see are not striking low enough and decelerating into the cue ball. The cue ball will only have backspin if you strike the bottom. Many beginners will aim low but then strike much higher as their tip reaches the cue ball. The result is no spin.
The solution to this is too stay down after the shot. Where is your tip? Is it pointing at the cloth or has it risen up?
Decelerating into the cue ball is also common. Beginners often start the strike too quickly so that by the time the tip has reached the cue ball it is slowing down.
Try and start the strike slowly and smoothly. Accelerate the tip into the cue ball.
If you strike low on the cue ball with a confident stroke then you shouldn't have any problems with backspin.