Hi Amelia,
That you posted the problem on here shows you are concerned enough to go back to the shop and ask them for help.
A good shop will have a look at it for you. Some zoom lenses actions are quite stiff at particular points due to the mechanism, and that modern optics quite often rely on 'floating' optics ie bits of the internal glass that move at different speeds/rates when the zoom control is used.
Ask the shop if it is common with this lens, and more importantly ask them to show you theirs in the shop (ideally not the demo one)
I'm sure they'll be happy to help.....don't be suprised if they want to sell you a protective filter for it if they dont see one of there too.
good luck
P.S sorry didn't spot you bought from warehouse express. Still worth trying a high street retailer if you can spot they have the same lens for sale.