Are you sifting the henna really well before you mix it?
Try this. After mixing place a small plastic bag with the corner pointing down into a glass turning a cuff of the plastic over the edges. Then take a knee high stocking and do the same. Put the henna into the stocking pushing it down into the bottom of the stocking and then pull both the stocking and the bag together up and out of the glass. Hold the bag tightly above the henna and pull the stocking away from the bag. What you want to do here is to keep pressure holding the plastic bag as you pull the stocking so that the henna squeezes through the stocking and into the bag.Then twist the bag closed with a twist tie. You can then clip the tip of the bag and squeeze the paste into whatever applicator you're using.
Are you using a bottle or a cone to apply the paste? Perhaps the opening is too large?
Do you put oil in your mix? Sugar? Sugar is meant to make the paste smoother and flow better.