Certain muscles can be underdeveloped which make it difficult. Before i became serious at the gym i had developed some strength but I had neglected key compound exercises like pull ups, bent over rows etc.
Atm i can bench 70kg x 5
squat 120kg x 5
dead lift 100kg x 5 (Dont really do this ex much)
I'm not strong yet but i'm definitely not weak however despite this i struggle to do 2 pull ups lol. I weigh about 80kg around 15% body fat. My lats are undeveloped and I really don't know why its so hard for me to do pull ups, I can manage press ups though. When you weigh 80kg+ working with your bodyweight becomes rather difficult