The black mark, sometimes sticky is caused by (in my case) a carpet which had been treated with scotch guard, but usually kitchen or damp areas can cause it.
I have a lady calls me every 3 months saying the vac is hard to push - again. I clean the sole plate with meths and it's like a miracle she says ! And thats a new sole plate too.
Has your hepa filter got the seals fitted in the orifice ? majority i see dont...