well i see that you are using vluzacns tools, so heres what influences your shadows: the sunlight itself, the skylight and the bounced light, which in vhlt also depends on the brightness of nearby textures... theres quite a lot you can try, you can increase the skylight by adding _diffuse_light under light_environment, to set a custom skylight for example 255 255 255 200. However that will make the "good shadow" lighter as well. You can replace the nearby textures with brighter ones, like if choose a ground texture thats brighter then more light will illuminate that corner. The true solution to this is HDR, the term itself doesn't make much sense but you know what it means. Of course HL doesnt have HDR so you would have to fake it but adding a bunch of weak lights, the more you add, the more natural it will seem, you can use as many as you want because they dont count as entities. If you think its too manual and is unrealistic, youre wrong, because its still closer to HDR than not having it. In most cases they go the easy way and just add a lamp or so... You can try one more thing as well, put a big block in that corner, make it a func_wall_toggle with start off and add a value zhlt_customshadow with value of lets say 1.5, that will fake HDR quite well. What it will do is amplify all light that passes through it by 1.5, preserving your "good shadows" as they are. If you use zhlt_customshdow, you might also need to add -customshadowwithbounce and -rgbtransfers to RAD.
http://zhlt.info/command-reference.html#hlrad