Stripes at colored thread

Some parts of the thread isn't colored, creating 'stripes' in the embroidery

If the two feeding wheels (E) is not closed thoroughly, the thread might slip a short while, hence causing a small part of the thread to be mis-colored.
Please turn the knob (B) forth and back a couple of times and make sure the feeding wheels are fully closed.

Another possible reason is that the Thread tension knob (D) isn't properly tightened. The tension shall be very tight. A good rule of thumb is to turn the knob all the way to the bottom and then release it 3 laps.