H-bridge with 7815 regulator
You must measure or lookup the motor DCR and see if the V+/DCR exceeds the current or power rating of any component cascaded from source to bridge.  

Boosting the voltage in a DC-DC converter also boosts the input current load.

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This is a simple problem of impedance transformation and underestimation of surge current. Surge current is 8 to 10X rated current =W/V determined by winding DCR which is transformed assuming 100% efficiency of DC-DC converter to 24/15^2 across 15v regulator.So your power source is trying to source about 25x the motor rated current. Next time , do the math