One reason that comes to mind is that your daily budget limit will often prevent impressions on keywords that have search volume. Do you have an adequate budget to purchase all the clicks those 300,000 keywords might generate within a 24 hour period? That would be the first thing I would check.
Check your impression share percentages that will give you a clue. Try setting your budget to about 3-5 times what you think it would take to purchase every available click on all your keywords.
