HELP!!! My ballast does not work properly when i plug it in to my timer...

Okay, I have just upgraded from 250 watt MH to 400watt.. the 250 was working just fine with my timer. (Brinks Timer Model 44-1010) It can support up to 15A. Now that i have moved up to 400 mh with a new ballast, it will not work at all... Everything comes on and functions correctly when plugged in to the wall socket, but not when in the timer... ???? I am no electrical whiz and would love some help if anyone has any input.. The system is plugged straight to outlet now but i am not always home to manually turn the lamp off so my plants can sleep... Anyone help me???

u need to buy a digital timer . cheap timers dont last 5 mins .. its not the running wattage its the initial start up current ..but a digital timer will be fine u can get them for about 6quid