I had my old shower replaced approx 4 months ago as it suffered from occassional heat loss (once or twice a month). Last week the new shower did it again - i.e. everything fine in terms of water flow but no heat whatsoever - but now it seems fine??
SOUNDS LIKE, LACK OF WATER PRESSURE AT CERTAIN PEAK USER TIMES, IE WHEN EVERYONE IS USING WATER IN YOUR AREA, THE BUILT IN PRESSURE SWITCH IS NOT SEEING ENOUGH WATER PRESSURE, THUS NO HEAT.