You need to have good timing to gain height with space jump. It's modeled after Super Metroid's space jump (adjusted for screen size in ZC, of course). If you're still having trouble, look near the top of the script file (it's all in the buffer and can be either edited there or exported) for these three constants.
const int SPACE_JUMP_LOWER_BOUND = 10; // How many frames Link has to fall before Space Jump can be used.
const int SPACE_JUMP_UPPER_BOUND = 40; // How many frames Link can fall before Space Jump use is barred.
const int SPACE_JUMP_PERCENTAGE = 0.7; // How high Space Jump is compared to normal jump.
You can fiddle around with those if space jump still seems too hard. Setting the lower bound to 0 and the upper bound to some high number that'll never be reached like 65000 will make it easier. It still has the restriction that you can't use it until you start falling, though. For a jump that can be used any time, even on the ascent, you might want to look at the MultiJump item script.