Hello,

They didn't decrease the drop rate of girl. It's just that in the new system, the more you have collected bosses' girls (aside the event girls), the more difficult are the next girls to drop (but it should never be significantly lower chance than the previous system).
So it's normal that it's fast at the beginning and get slowlier later on.
The dev team said that for a regular player, a full month without dropping any girl (aside from event girls) shouldn't happen anymore. (While before thatit could take up to 2 months).