LM2596S-ADJ Buck Converter Circuit heating up and wasting a lot of voltage somewhere
The datasheet for the LM2596 gives the following formula for the output voltage of the adjustable version:

\$V_{OUT} = V_{REF} \cdot (1+\frac{R_2}{R_1}) \$ where \$V_{REF} = 1.23\$

You have R1 as 10k ohms. Assuming the max value of R2 at 100k, the maximum output for your setup is 13.53 volts. Perhaps that is why your voltage is not approaching VIN. Try swapping out the 10k resistor with the recommended 1k ohm value.