Boost Converter - Input follows Output
Crudely said, a boost converter tries to get the input voltage up to the target output voltage and a buck converter tries to get the input voltage down to the target output voltage.
A boost converter therefore fails to get a voltage up/higher when the input voltage is already higher than the target output voltage.  

The output of a boost converter is given by
$$ V_{out} = V_{in} \frac{ 1 }{ 1-D } $$ where D is the duty cycle for which applies 0 &lt; D &lt; 1. You can see this duty cycle at the mosfet's gate and/or at the switching node.
When the \$V_{in} &gt; V_{out} \$, in order to obtain the target output voltage, according this equation, D &lt; 0. If e.g. Vin = 28V and D could be -1, then Vout=14V. However, D cannot be smaller than 0. So, the mosfet stops switching (which is 0% duty cycle) and the inductor could be crudely approached as were it a wire.
And therefore, the ouput voltage follows the input voltage.    

If you want a converter with an output (e.g. 14V) that lies within the range of the input voltage (e.g. 7V to 18V) you need another topology, e.g. a buck-boost converter.
Wiki about the types of SMPS