If this is happening on a gun or pistol with a Hop-Up then a small adjustment will fix the problem, please see your gun’s manual for information on how to just your Hop-Up.

If it is not a Hop-Up issue then how about looking at one of these possible issues:

Abnormality of BB pellet
Avoid using poor quality or deformed BB pellets. Use high quality, seamless and perfectly round BB’s.

Abnormality in sectional parts of the barrel
Replace with new parts if there are scratches or deformities in internal parts of BB chamber/inner barrel.

Synchronisation failure between moving parts
Lubricate slide, frame-assembly, gun barrel and other movable parts with oil or clean with warm soapy water and re-lubricate.

You have lubricant on the Hop-Up mechanism 
Clean your Hop-Up and inner barrel with correct products.

Holding and gripping gun incorrectly
Try to avoid wrist or handshake, grip tightly when shooting. Grip gun with both hands where possible.