I agree with your first assessment, and with Winger et al; it sounds as though your developer is cooling down during your sessions. There's no need to extent your print exposure times. You should extend development time as the temperature of the dev decreases; the packaging may tell you to develop for two minutes at 20 degrees but at (say) 10 degrees you might need (say) four minutes development time. Unlike film developers, print developer times are a rough guideline. Extra development time won't adversely affect prints because you should develop prints to completion - until the print no longer darkens. If you 'pull' the print (remove from the dev before fully developed) you'll have insipid blacks and poor gradation in the mid-tones.
The other possibility is developer exhaustion; if you're diluting as per instructions you should be fine. If you do long sessions with lots of prints, replacing half the dev with fresh solution should fix that problem.
Be patient and slow down in the darkroom - your prints will improve no end. There's no race to produce prints quickly. Print processor machines are a convenience and can produce really lousy prints with poor contrast when they want (been there...), especially when the developer gets exhausted, so don't use your machine-processed results as a benchmark.