flush_rewrite_rules() still not work for single_template against permalink

guys, i’m developing simple commerce plugin and using register_post_type. Like other closed thread, i’ve found problem that need fixing.

when i using plain permalink, my filter ‘single_template’ works like a charm but when i change permalink into else it show the 404 page.

in other thread explained, i already try using function flush_rewrite_rules(); after i call the register_post_type but again, it didn’t works at all.

is there any solution that i missing here?