LM3488 step up 12 to 24 V

I use LM3488 boost topology.

Vin 10.5 to 15 V from VRLA battery (12 V).

VOut 24 V ,  Iout 3.1 A

I'm having Vout fall to about 18 V when maxim current drain (Iout).

My question :  What are the critical items to review in project  to get effective 24 V at 3.1 A ?

Thanks and Regards