7 Segment display driver issues
You obviously figured out your issue via the comments, but for those finding this question in the future, the answer lies in the Nexys datasheet. In short, yes, the 7-seg display digits are multiplexed, and as such, there is a setup and hold time when interfacing with those common anode/cathode transistors.

In this case, however, you have to switch between them slow enough for our human eyes to see the light, but fast enough so that there is no flicker. Think PWM: if you're switching between digits too quickly, the light will be dim.

The Digilent data sheet for the Nexys board answers this exact question. Check out section 9.1 which talks about interfacing with the seven-segment display. In particular...


  Each digit is illuminated just one-quarter of the time, but because
  the eye cannot perceive the darkening of a digit before it is
  illuminated again, the digit appears continuously illuminated. If the
  update or "refresh" rate is slowed to around 45 hertz, most people
  will begin to see the display flicker. 
  
  In order for each of the four
  digits to appear bright and continuously illuminated, all four digits
  should be driven once every 1 to 16ms, for a refresh frequency of 1KHz
  to 60Hz. For example, in a 60Hz refresh scheme, the entire display
  would be refreshed once every 16ms, and each digit would be
  illuminated for ¼ of the refresh cycle, or 4ms. The controller must
  drive the cathodes with the correct pattern when the corresponding
  anode signal is driven. To illustrate the process, if AN0 is asserted
  while CB and CC are asserted, then a "1" will be displayed in digit
  position 1. Then, if AN1 is asserted while CA, CB and CC are asserted,
  then a "7" will be displayed in digit position 2. If AN0 and CB, CC
  are driven for 4ms, and then A1 and CA, CB, CC are driven for 4ms in
  an endless succession, the display will show "17" in the first two
  digits. An example timing diagram for a four-digit controller is
  provided. 
  
  
  
  Source: "Nexys 3 FPGA Board Reference Manual". Digilent Incorporated. April 11, 2016. pg 19. PDF.


So there you go.