1. Don't stay on the same program for too long.
It's important to switch up your programs after about 4-6 weeks, you'll stop seeing strength gains, power gains etc after that. It's important to give your body progressive overload specific to your goals.

2. No structure in your program
If your program has no structure, and you're just doing whatever you feel like at the moment at the gym, you definitely won't see progress. Performing specific exercises for a period of time is important, structuring it out so you're not hitting the same body part a few days in a row etc. There are many programs online that can help you reach your goals or contact a personal trainer to get a program specific for what you're looking for.
3. Neglecting your Nutrition
Putting in the effort in the gym isn't going to do anything if you're eating like garbage. You can't out train a bad diet. We've all heard that saying a bunch of times but it's true. Now that does not mean that you can't indulge every now and then, having a few things to look forward to can help you stay on track throughout the week.

4. Not Getting Enough Sleep
Another reason why you're not seeing results is not getting enough sleep, if you don't sleep enough your body will never recover from the exercise you've done. Your muscles tear everytime you work out, small micro tears, and they require rest to recover.
5. Not Managing your Stress
If you're body is constantly fighting stress you won't be able to recover from your workouts. You also won't be able to perform as well, if you're very fatigued you won't be able to lift as much, run as fast etc. Taking some time to decompress and relax will help you perform at your best.