Whats up with my ollies (skateboard)?

ok so i can ollie like really high actually but when im moving and i ollie i dont even pop O_o half the time and its pissing me off cuz ill stomp on the abck of the board and i wont pop and ive done this gap before that a good size but i cant ollie off a 3 inch ledge why is that and when im moving my ollies arent good at all eather only when im going like 1 push slow i can ollie and ive been trynig my ollies for 2 months and this always happens T_T