The Common Cathode resistor is limiting the current
You need a series resistor at the anode of each LED segment, not a single resistor at the common cathode. Here is a schematic representation of a single seven-segment display:

With your current circuit, you've connected a single resistor to the common cathode. That will cause the current to be dependent on the number of segments which are lit at a given time.
Each LED will drop approximately the same forward voltage. For the standard red indicator LEDs in most 7 segment displays, that's about 2 V. Assuming that you are using a 5 V driver, that leaves 3V to be dropped over the resistor. Ohm's law says that the current through that resistor is given by:
\$ I = \cfrac{V}{R} \$
or about 13.6 mA. This 13.6 mA will be constant regardless of how many LEDs are lit. If you light all 7, the current in each will be 13.6 / 7 ~= 2 mA. The current in an LED is proportional to the luminosity; 2 mA is barely visible. To fix this, you need to connect a resistor to each anode of the LEDs. This will allow for 13.6 mA in each resistor which does not need to be shared. The current through the common cathode can then be 13.6 * 7, so you'll get full brightness.
An alternative mechanism
That is a lot of resistors, however. If you're using through-hole resistors, it could increase the size of your circuit considerably. You can save board space, soldering time, and component purchasing cost by writing some slightly more complicated software.
An alternative control mechanism would be to only illuminate one segment at any given time. By blinking (or not blinking) each segment at 200 Hz or faster, your eyes will see the average. Because the LEDs will only be illuminated for 1/7th of the time that they would be with individual illumination, the average luminosity will be less and you'll want to increase the current. Over 20 mA or so, these LEDs won't increase in brightness appreciably so just use a 150 ohm resistor.