Unable to thicken mirrored surface and other issues with thickening

Hello,

I am unable to thicken and merge two surfaces.

In the attached model, the surface knit( named as "unable to thicken") is the surface causing issues. But a similar surface(named as "can be thickened") can be thickened. I have tried mirroring the surface I can thickened but have been unsuccessful. Please help.

Many thanks in advance,

samio roi

SolidworksSurfacing