Why isn't my 5-to-24VDC boost converter supplying enough current to turn a 24 VDC motor?
A 5 ohm resistor will try to draw a current of 4.8 amperes from a 24 volt power supply.  You say you measure currents of 280 mA and 400 mA with 2 different supplies.  Clearly neither supply is providing 24 volts. If you want to test your supplies at the same current level as your motor, you need a much larger resistor (24/0.18 = 133 ohms). Also, have your measured the voltage provided by your power supplies when driving your motor? It is clear that neither one is putting out 24 volts. If your supplies pass the test with a 133 ohm resistor, then I suspect that the problem is the inrush current of the motor which can be many times higher than its running value. Your beefy supply can handle it since it can provide 2 A which is more than 10 times the running current. Your other supplies cannot do this and are shutting down in some fashion depending on their design.