@rania To add to the other comments re. inefficiency/eating enough..
Single arm exercises with dumbbells might be a better idea for you looking at what you're doing, as a thought.
Single arm is more demanding, but the dumbbells will also let you go up in smaller steps also because it looks like you're stuck adding circa. 2.5 - 5 kg i'm guessing with fixed barbells mostly. A smaller gap to the next step means you will probably be able to eek out sufficient reps to push yourself forward a bit more.
You might find your smaller stabilising muscles will struggle. Something like a chest press machine is quite isolated and won't hit as much as something like a free weight bench press variation. Make slow movements (i.e. push strongly on a press, but then lower down over more like 3s) which will add time under tension. By working even the smaller components your overall base strength will increase.
I'd also consider mixed rep ranges as well - sometimes 6-8 just high weight to hit strength, and tend to push into higher reps for volume at other stages. 15 reps is quite high when you're trying to up weight, you would probably find 8 reps or so sufficient as it will enable you to express a bit more knowing you only have half the reps. As you progress you can mix in some at higher reps for the volume and hypertrophy. I've got a trainer to sort it out, but programs might have a framework for you.