How to calculate the feedback resistors for an SMPS
The basic principle for the feedback resistors is, that it is a voltage divider.  With 5.6k and 1k as shown, the junction between them will show a voltage based on the ratio of R17 and the total.  5.6k+1k is 6.6k, so the result is 1k/6.6k, or 0.15.  That means that with 4 volts applied, the junction will be 4 * 0.15 or .6 volts.

The regulator (when it's working properly) will adjust the output voltage up or down until it sees .6 volts on the feedback (FP) pin.  You can test this by placing a somewhat higher resistance across R15 or R17.   10k would work here.  If you put it across R15, the voltage at the junction will rise, and the regulator will lower its output voltage to compensate.  Likewise, putting it across R17 will lower the feedback voltage, and the regulator will raise the output voltage to compensate.  If you get no reaction, you have a different problem.  Sometimes the supply/regulator has got the voltage as high as it can go, but in that case, the voltage on FB would be correspondingly low.

And sometimes, the resistors are just wrong, or there's a miswiring, or there's stray resistance somewhere (like a shorted capacitor).  But the basic voltage divider principle can't be violated. There's always another explanation.