I'm guessing you set the shot count to 5 rather than 20. I find that screen madingly confusing, it will make me break down and get a remote with a timer function just to get around that one screen.
- On the first menu, you need to select NOW or the time you want it to start
- On the next menu, you need to select the interval time, in your case zero the 1 in the minutes(') column and then put 5 in the seconds column(")
- On the next menu you set the number of intervals (times to shoot) times the number of shots to take each time (shutter releases for each interval)
So in your case, set it to NOW, then 00.00.05, then 20 and 1.
That will give you intervals of 5 seconds, twenty intervals and one shot at each interval. That should result in 20 total shots.
Just read that last line again. You were shooting at 20 seconds, so if you set the interval to 5 seconds, the shutter open time is over running 4-5 intervals. You would need to set the interval time to 26 seconds to get a five second pause between the 20 second exposures.
You may have had the rest set correctly, but the interval time is a hard time, it doesn't stop counting for shutter open time. You have to add the long shutter open time to your interval to make it work.
According to Thom Hogan, the actual shutter open time for a 20 second exposure is 21 seconds, so plan accordingly.