Rule out a change in demand first
Think about what changed: another person in the house, longer showers, laundry or the dishwasher running during morning showers. The number that matters is the heater's first-hour rating, not the tank size. PNNL's Building America guide to heat pump water heaters explains that this rating is how many gallons of hot water the heater can deliver in one hour starting from a full tank, and that it's printed on the heater's EnergyGuide label. If your busiest hour now uses more than that, the heater may be working fine and simply be too small for the way the house uses it.
Note how it fails
Write down which of these you see. It gives the technician a starting point instead of "not enough hot water."
Water starts hot, then fades early in the shower
Water never gets as hot as it used to
Hot water comes back, but takes much longer between uses
The change was sudden, or crept in over months
It started after plumbing work or a setting change
In the manufacturer troubleshooting chart for one electric model, included in a filing posted by the Department of Energy, "insufficient hot water" lists a failed lower element, a temperature set too low, sediment or lime in the tank, a defective dip tube, a heater too small for the job, wrong piping connections and leaking faucets. Slow recovery is listed separately, under a failed upper element. Other models differ, but the pattern you note helps narrow it down.
If only one shower is weak while other taps stay hot, mention that. It points the plumber toward that fixture before anyone blames the tank. Our guide on low water pressure at one faucet covers similar one-fixture clues.
Two checks that are safe to do
Check the delivered temperature. PNNL's water-heater temperature guide uses the faucet nearest the heater: run only the hot tap for about three minutes, fill a mug and put a meat thermometer in it right away. Use a heat-safe mug, keep children away, and skip this if handling very hot water is a risk for you. PNNL notes that EPA recommends 120°F to prevent scalding, so don't crank the dial up just to stretch a shower. Give the reading to the technician instead.
Look for leaks without opening anything. EPA's WaterSense home maintenance page suggests regularly looking over the heater for leaks, worn gaskets, loose connections and corrosion. The troubleshooting chart also lists leaking faucets and pipes as a cause of short hot water, so mention any dripping hot tap. Don't remove covers, touch wet equipment or open the drain valve.
Philadelphia clues: white specks and hard water
If you're finding white or gray bits in faucet aerators, the Philadelphia Water Department's drinking water FAQ says a common source is the water heater's dip tube, a plastic pipe that can break apart and send small pieces to the faucet. A defective dip tube is also one of the causes in the troubleshooting chart above. If you're on Philadelphia water, the same FAQ calls it moderately hard, typically about 6 to 9 grains per gallon, so sediment and lime in the tank are worth asking about too. Bring a few of the specks in a bag to show the plumber.
If you have a heat pump water heater
Check the mode on the display before assuming a part failed. The Department of Energy's water heater buying guidance says that in efficiency (heat-pump-only) mode, recovery is slower, the first-hour rating is lower, and there may be times when demand isn't met. Hybrid mode, the default on most units, mixes in the electric elements to match a standard electric heater's output, and some units have a high-demand mode for busy stretches. Use your manual for the exact mode names.
What to ask when you book service
Give the heater type (gas, electric or heat pump), the model number from the label, your temperature reading and your notes. Electric element and control testing and any gas burner work are for a qualified technician, not a homeowner. Then ask:
What test showed which part failed?
Is the heater putting out the hot water it's rated for?
Does the tank need flushing, and is that right for this model? (EPA suggests yearly professional service.)
If you recommend replacement, how does the new first-hour rating compare with our busiest hour?
If replacement is the answer, check whether the job needs a permit with our guide to Philadelphia L&I plumbing permits for a water heater swap, and use our checklist for hiring a home repair contractor in Philadelphia and Bucks County before work starts.