The Philips Avent Niplette is here to help you out. If it keeps falling off there might be a simple solution. Find out how to keep your Niplette in place.
A small smear of lotion or cream at the base of the Niplette will improve the seal and suction
The tube may be blocked by discharge from the nipple or by the lubricant. Flush the tube through with hand-hot soapy water by drawing the liquid up through the thimble and tube into the syringe, and then flush back out. This exercise should be repeated a number of times and then finally with clear water.