Factors that effect what current a Synchronous buck enters the Discontinuous mode
A buck converters enters DCM when
$$\frac{2Lf}{R_{load}} &lt; 1-D$$
where
f is the switching frequency
D is the duty cycle
L is the buck inductance
One can replace \$R_{load}\$ by \$\frac{V_{out}}{I_{load}}\$ and one gets the condition
$$\frac{2LfI_{load}}{V_{out}} &lt; 1-D$$

How can [a buck converter] then enter discontinious mode at different currents and what are these different factors that effect this current

If the duty cycle changes, for example, due to a change in supply voltage, then the converter may enter DCM even though the load current remains constant.
Also, there are some controllers that may alter the switching frequency, with a similar effect.
If \$V_{in}\$ or \$V_{out}\$ or f changes, then the converter will enter DCM at a different current level.

how exactly would I push the SMPS into discontinuos and continuous when the output current is fixed?

By changing the duty cycle (or frequency) by changing the input voltage.