Shutil.copy not working after use os.walk()

Question

I have tried to run the code with the fix path at the beginning. After it able to work, I change it to walk through directory (folders & sub-folders). Now it doesn't run with error, but the shutil.copy line doesn't copy the files.

BEFORE:

for i, dwgfile in enumerate(newdwglist, start=0):
    dwgfile = newdwglist[i]
    os.chdir('C:\\Users\\chinhing\\test folder\\source folder')
    if os.path.exists(dwgfile):
        shutil.copy(dwgfile, destlink)
    else:
        i = i + 1
        missdwg.append(dwgfile)

AFTER:

   for i, dwgfile in enumerate(newdwglist, start=0):
        dwgfile = newdwglist[i]
        for roots, dirs, files in os.walk(mainpath):
            for dwg in files:
                if dwg == dwgfile:
                    if os.path.exists(dwgfile):
                        shutil.copy(dwgfile, destlink)  #notworking
                        print(os.path.join(destlink, dwgfile))
                    else:
                        missdwg.append(dwgfile)
                else:
                    i = i + 1