Cyanotype developing times

I have a small problem with one of my negs. The neg is a 4x5, quite dark b&w Ilford FP4+. I have made something like 30+ cyanotype pictures with it, but only 2 have been acceptable. I dont have a uv source other than the sun. I have been trying to time it according to the my notes from all the developments. But they turn out too light or too dark. Only 2 perfect ones yet. The one day I produced most of the prints, the Sun was shining from a clear sky. According to my notes, going from too light to too dark is a matter of 15 seconds. What I am doing wrong ? I have do quite a lot of other cyanos that have turned out perfect, but this one picture insist on giving me a hard time.

That seems like an incredibly short length of time for a difference, especially for a dark neg. Are you using a splitback frame to check exposure or are you just doing timing with the sun? I wasn't very keen on timing with the sun because it's not very consistant around here, one cloud moves across and throws off all the calculations.
Maybe it's just a problem with the compressed scale... The only time I've had the problems you describe is with thin negatives with little contrast.

All the pictures have been done this far without a splitback frame. My first splitbackframe just arrived last week. I love to just guess if it is done, and have been lucy so far with all other pictures I have done. It is just this one that annoys me. I have been thinking about a small facial tanner like You wrote in another thread, but for now the sun has to do. I know it is not that realiable. But it also the fun part of the process.

I have also just got new chemicals, but have not printed with them yet.

Have you changed the paper?

All the pictures have been printed on Hahnemülle Brittania paper. I have tried both sides o the paper.

I use the sun to expose cyanotypes, and find that a split-back frame is essential, due to the previously mentioned variations of light levels. That said, I've also found that not all paper give acceptable results with all processes. My favorite paper for cyanotype if Strathmore 500 plate finish, although I was told by Sam Wang a while ago that they changed the formula and this paper no longer has the wonderful qualities that it used to have. I have a stockpile still, from an art store sale a few years ago, so I have not bought any new to see the difference, but the moral of the story is try different papers as well, as they all give different results.

- Randy