I seem to be battling to repeat results with prints. Funnily enough this is not with burning and dodging but with the initial exposure. For instance the other night I made a print which had the initial tones and exposure that I required. I wrote down the exposure times, enlarger height, f stop etc. The next night I decided to do some more work on the print. I set everything up according to my notes, replenished the developer/fix and made a print. The contrast was completely different. Very weak blacks and a general underexposure. I made a repeat print as I thought that maybe I'd made a mistake setting the timer but got the same results. I found that I had to increase the exposure on both grade 0 and 5 to get the same starting point as the previous night. Being a beginner I'm quite fastideous about time, temperature etc. as I don't want that to be a further variable in my learning curve. This is not an isolated incident. It's happened a few times in the past couple of weeks. Could the enlarger bulb be causing this by taking time to warm up or is it a problem with the filtration unit on the colour head? The enlarger is a Durst Mod70 with the standard colour unit. It's also the same paper stock.