I may be way off but in the pics it looks like the output for your Ehiem would be causing a current to flow across the top of the tank down the front to then go straight into the front of the caves.
Maybe try moving the output to the end of the tank (side) to change the current to go across the front of the cave and the length of the tank?
I'm sure I be corrected if wrong on this thought just know that with my BN's they do not like the current flowing into the cave but past it instead.
thank you Zoban, in fact i understand the point that you say about the current. however, in this tank also have another eheim 2212 on the top left of the tank which will create the current from left to right cross the front of the cave as well. ( it is not there in the picture, my bad) so if the current really effect the zebras on moving in then i will turn the spray head up. this way the current will come only from ehiem 2212 on the left top of the tank only.
i try with different spot already as you can see in my old tank set up post. (at the end of left side and right side) however, it still doesn't wok so i try the same way i saw from McEve set up. so now i want to know is the angle of the cave is the reason or something else. i don't want to stress them too much by moving around the cave til they move in. so are they usually move into the cave right away? or i have to wait for a couple week.
if they move right away then i guess i have to keep moving things around.
by the way any other possible reason on this except the angle and current?
When I set up a new tank I wait for two weeks to see which cave they move into. If a cave is left I move it a few inches, not much, just to face the opening in a different angle and see what happens