Surface problem?

Hi guys,
another problem I’ve this function

SDL_Surface *grabframe(SDL_Surface *source, SDL_Rect grabr, Uint32 cols,
Uint32 frm)
// crea una surface delle dimensioni della tiles e del formato sorgente
SDL_Surface *temp = SDL_CreateRGBSurface(SDL_SWSURFACE, grabr.w,
grabr.h, source->format->BitsPerPixel,
source->format->Rmask, source->format->Gmask, source->format->Bmask,

// ottengo x e y da passare alla SDL_Rect
grabr.x = grabr.x + (frm % cols) * grabr.w;
grabr.y = grabr.y + (frm / cols) * grabr.h;

// blitto sulla surface temp
SDL_BlitSurface(source, &grabr, temp, NULL);

// ritorno la surface ottenuta
return temp;

and this work fine when I make a virtual screen with my tiles
I have this function too, for the sprite

SDL_Surface* S_show(SPRITE *sprite)
return grabframe(sprite->sf, sprite->rt, sprite->am.cols, sprite->am.att);

when I call the S_show into my main loop the image is not drawed…

anybody can understand why?