shadow darkness in restart leve

hi.
I use unreal engine 4.12.5;
on first time of starting game all shadow is smooth and correct but when I do restartlevel all shadow become dark.
how can I fix this problem.
my light is dynamic and I use Ray Traced Distance Field shadow/ambient Occlusion.
on first time:

105624-shadow02.jpg

after restart :

105625-shadow01.jpg

please help.