Passengers tend to cancel scheduled rides frequently, so Skeddy assigns overlapping rides to you in order to provide a backup ride in case other rides are canceled.
At that, when the number of credits used for the assigned but not yet completed rides exceeds your credit balance, Skeddy stops searching for new rides, and you are alerted to buy more credits.
To have Skeddy always searching for new rides, It is a good practice to keep your credit balance double the number of credits used for all your assigned rides.