It sounds like you're not entering the aerial with enough spin. The 2 basic ways to 180 is via rotation and via counter-rotation. You body in the air can't spin by itself. So it needs help.
Rotation - you INITIATE the spin while you're still on the ground. With proper timing, your body will rotate 180 while you're in the air before you land.
Counter-Rotation - you twist your body and arms arround 90 degrees before the air. While in the air, you release the twist 180 in the opposite direction...and your board will rotate opposite of that 180. This applies the Law of Conservation of Momentum. Because your top mass rotates 180 while your bottom mass rotates -180. Actually, that's simplified for understanding, but it really depends on your specific upper body how much you have to rotate to get the bottom half to rotate -180.
When you're good enough, you can mix both techniques while in the air to make it more acurate or perform this trick called a "Shiftie".
OMG, not the whole counter rotation thing again...
Can you please stop giving your so-called 'advice' on riding techniques? Most of us here know that your explanations are bullshit, but you could do real harm if some newbies were to follow your 'instructions'