Storm windows really aren't (or shouldn't) be designed to be airtight. There should be weep-holes at their base to allow condensation that drips down the window to drain off of the sill and often there are small baffled slits at the top of the window to allow moist air to escape. Storm windows are really designed to keep wind and rain from blowing directly on your double hungs/casements or other opening windows, thus extending the windows life and preventing direct drafts or leaks.
But to your question. If the SW is aluminum, there is often a rubber seal in a track or groove that should be replacable. Some I have seen have a rubber or neoprene seal glued to the inside of the sliding track. I've even seen foam tucked up into the top of a track that can simply be pulled out. Your best bet is to go back to the manufacturer or distributor and see if replacement seals are available.