It's most surely caused by your low speed needle being out of adjustment.

I'm a big fan of the "pinch" test for setting the LSN.

Engine warm and at idle, pinch the fuel line ahead of the remote needle (if you have one).

If the engine shuts down, you're too lean.

If the engine does nothing, you're too rich. You can verify this by pinching and holding the line..... The more rich you are the longer the engine takes to increase speed.

The engine should increase speed slighty, that's perfect! That should get you very close. If you still have a small problem with transition, refer to the instructions mentioned by others for tweaking after the pinch test. But mine's always been dead-on.