Could be a very tricky problem to resolve.
I have just a few ideas & reminders to share which could help:
1) While using the "surfacing" tools, the blue lines indicate a gap between the surfaces. This is a visual aid for when you need to knit surfaces.
2) While using the "Knit Surfaces" command, try moving the slider to allow larger gaps to be knitted; there are two sliders.
3) Consider revising/replacing "coincident" relationships with "pierce" point relationships when applicable
4) Knit the surfaces as you go, instead of trying to merge them all in the end
5) Use the "Filled Surface" command, it can be a modeling-life-saver
As always, hope this helps :)
somewhat helpful thanx mate much appreciated it took some time but I had to reboundary some surfaces