This complaint is different from, "No Heat." Let's define exactly what this really means. A normal load of laundry takes about 40 minutes to completely dry. Heavy towels maybe 60 and light underwear only 30. A customer will notice that they need to restart the dryer because the clothes are not done in the time expected or what they are used to. (I'm sure you have carefully checked the lint trap)
Trick of The Trade - Fabric softener sheets will slowly clogg the tiny holes in a lint trap. Hold the trap up to the light for careful inspection.
With rare exception, it means a clogged vent.
Creature in the Vent
Clogged Where Not Accessible
The surest proof of this diagnosis is to get the customer to run a load of laundry with the vent disconnected from the back of the dryer. I guarantee it will dry in record time.
If it does, get the customer to call a vent cleaning service or pull down the entire duct system themselves and clean or replace it. Customer always say, "I always clean my lint trap." They are seldom aware of the 5-40' run the 4" duct makes before getting to the outside flapper. It is helpful to look outside and inspect the flapper. A blocked or damaged one will kill air flow. Vent clogging is a chronic problem in new houses with upstairs laundry rooms that must feed the vent all across the house to exit. Clogging represents a fire hazard. Get to know the local dryer vent cleaning guy; he will be happy to send you a nice referral fee!
Dryer Vent Wizard
I personally find all but the shortest duct line a real pain to fool with and gave up years ago. It requires ladders, a vacuum cleaner, a lot of time, plus you will get filthy. A duct that disappears into a wall or ceiling is beyond your capabilities anyhow. It's up to you, but you can make easier money elsewhere.
The Vent is Wide Open, What Next?:
Normal drying requires several factors working in tandem. If any one is not met, there will be problems. Adequate heat, no internal cold air leakage within the dryer and the correct amount of well spun clothes:
Inadequate heat input. Possible Causes:
Bad gas coils. For complete description see my gas valve video set. Coils often provide heat for a short time and then quit mid cycle.
Universal Gas Coil Set
Not enough wattage on the electric element (one of two elements blown on a GE dryer; the only brand with two elements) Electric element operating on 110vac instead of 220vac (an apartment, trailer park or amateur wiring scenario) Operating a propane conversion dryer on natural gas; not enough gas.
Leakage of cold air into the dryer after the heating unit input. Possible Causes:
Damaged drum seals, front or rear. Bad or missing door seal. Twisted or sprung door. Loose front of dryer. Damaged lint trap seal. Gummy lint trap screen from fabric softener tissues.
Replacing a Felt Seal
Abnormal amount or improperly spun clothes. Possible Causes:
Bad washer- dragging seal, bad belt or clutch system. See individual washer design quirks. Chronic overload by kids, visitors, renters or laundromat type folks. You should be unable to manually squeeze any water from well spun clothes. See "My washer is not spinning."