Could you be drying mixed loads? You said the timed cycle works OK. That means your heating element is probably OK and your vent too.

I got this from page 51 of the owner manual...

•Separate heavy items from lightweight items. Larger and
heavier items take longer to dry.
• Heavy fabrics take longer to dry because they tend to
retain more moisture. To help reduce and maintain more
consistent drying times for large and heavy fabrics,
separate these items into smaller loads of a consistent
size.
• Use the appropriate control settings for the type of load
you are drying.
• Make sure the lint filter is cleaned before every load. With
some loads that produce high amounts of lint, it may be
necessary to clean the filter during the cycle.
• Confirm that the exhaust ductwork is properly configured
and free of debris, lint, and obstructions. Make sure that
outside wall dampers can open properly and are not
blocked, jammed, or damaged.