The front 2 floor vents probably use space between floor stringers to channel hot air, if you can either use a flexible bore scope ($35. at harbor freight) or fish tape, find heater you'll see round ducts going to bath wall vent and to rear bedroom duct but 2 round ducts will turn down into floor with flat metal and silver tape sealing ducts to floor, they may be blocked somewhere. If that's what you find PM me I can walk you through a possible fix. If they use space between floor stringers turn on heater, you'll find the floor will be very warm feel it to trace the rout from heater to vent.
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