Do you mean Normal nail polish? If they are brand new they should be nice and runny.
So are they :-
Too old - check the opened shelf life
Someone has left lid off for too long and it's gone thick, could be during a very slow application service.
Lumpy -Dried bits from the top of the bottle have landed back in the bottle because someone can't be bothered to wipe the rim with acetone to keep it clean.
Stringy - Needs throwing out I'd say. Or perhaps it's an advanced wear system. Extra wear polishes or longer lasting top coats have a more polymer type fluidity and can act a bit stringy.