Why can't beclovent (beclomethasone) be used to stop an asthma attack in progress? i think it helps me when i get sick.

Preventative: Beclovent (beclomethasone) is an inhaled steroid to reduce chronic inflamation. It is a long term maintenance/preventative medication. Short acting beta agonists like albuterol are for the immediate relief of wheezing.

Rescue vs treatment: Using a steroid inhaler during an attack is not "useless", but is just not sufficient. Steroid medications take hours to days to work, and in an attack you need help right away.