Surface thicken problems in Solidworks

I think that there is a simple solution which you can try. just Un - tick the " Merge result" box and, retry your thickening procedure.
Best regards
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Step 1:
Click the relevant surface which you want to Thicken, and go to the surface Tab, and click the Thick box. And then Un tick the " Merge result" box and, retry your thickening procedure.