Since you just restarted your darkroom experience, I assume that the developer was purchased recently. That suggests freshness, but developer does age on the shelf, and it may be old. With the current lack of darkroom activity, some stores have stock that is years old. When mixed, that could go bad quickly.

I echo the comments about developing fully. I mix up Defender 54-D, and it takes one minute and fifty seconds to fully develop Multigrade RC. Check the instructions for the developer you are using, and develop at least as long as recommended. To determine exposure, you should make a test strip and develop it for the recommended time; then choose your initial guess for an exposure based on what looks good on the test strip. Five seconds is a pretty short exposure. Stop down one or two notches and give yourself time for some dodging and burning, if necessary.