DDR-SDRAM controler

I have found some strange behavior of DDRSDRAM-controler.

when power up my system, the core does not work good.

but, once I put reset signal in the SOPC builder.

it works well.

I thought this is matter of time.

so, I made counter which count till 1sec ( I tried 10 sec as well ).

and make reset signal.

but the reset does not work. no matter how long it waits.

then I put reset signal externally, it works again.

what happens in the SDRAM?