Dividing output voltage from a buck boost converter
Now that we've gotten your setup explained in the comments, we can explain your problems.


Switching power supplies and breadboards don't mix.  Breadboards have parasitic capacitance between pin rows, and parasitic inductance in the rows - not to mention in the wires.  The contacts are also often weak and don't hold well - the resistance varies just by moving the wires.  The contacts also are not made to have amperes of current flowing through them.  You really nead a PCB for a switching power supply.  At the very least, you need a carefully soldered circuit built on a piece of perfboard to get a switching power supply to work reliably.
A voltage divider is a poor way to generate a lower voltage from a higher one.  Your load changes the ratio of the divider.  The correct way to get what you want is to switch the feedback resistors so that the regulator generates the output voltage you want.


This section of the datasheet shows how:



Pick a single value for \$R_2\$, then figure values for \$R_1\$ that give your desired \$V_{out}\$ and use a switch to select which \$R_1\$  is connected to set your output voltage.