Why output power is low?

[ROSELJR]

Why the output power of LM338 is smaller than the power of switching power supply

The LM338 (TO-3 package) I used transforms the voltage to 5V, and the measured load requires 2.7A. When it is connected, the voltage drops directly by 1V, and the voltage fluctuates continuously after a few minutes. But I use a 3A switching power supply and there is no problem at all. Is the actual power of LM338 not enough?

[Spex]

I'm not an expert in this regulator, and I'm not sure how this question is connected to sbRIO or SOM.  Not sure you will get much traction in this forum.

What is your input voltage to the LM338?  The data sheet appears to require a >3V difference between Vin and Vout, so your input voltage to the regulator needs to be greater than 8V.  

For high Vin voltages, it appears the LM338 derates it's max output current as the Vin-Vout delta exceeds 10V.  

What is the circuit design? How have you debugged your regulator design? Have you loaded the power supply with different loads to characterize?