There may be too much Liquid White on your canvas.  Use this "fingerprint test" to make sure you've applied the correct amount, after brushing on the Liquid White, gently touch the canvas.  Your fingertip should show a nice impression of your fingerprint, not be completely covered in white, not have little or no trace of white on it.

Or, you may not have enough paint on your brush.  Remember, it is important to use a large amount of paint when accomplishing many of the Bob Ross effects.  Make sure that the fan brush bristles have a lot of Titanium White, loaded correctly by holding the brush upright and gently pulling it through the paint.

Tip from Bob Ross Inc.