lighting through alpha texture

Hello everyone,

Hope someone can help me - I am pretty new in Unreal, and am having a lighting problems.

I have a spotlight shining in the window which creates a really nice effect through the lace curtains in the preview, however on building, it becomes blurry and pixelated. The lace curtain is made using an alpha and I believe this is what is causing the problem? Is anyone able to advise if there is a way to make this effect work, or should I just ditch the transparency on the curtains?
Thanks.
3241ef3719f4ef59fb2f40720645f85e50665a6b.jpeg - preview of desired effect

- result after building

Try to use a higher lightmap resolution for your mesh.