Silly (maybe) question re imported module(s)

[Grizzy Adams]

Morning All

I'm working through the tutorial and running / saving work that I wish to keep 
and build on, most times I can save and (re)import later with no difference to 
when typed in console or editor and run with F5 (which saves before it can run) 


But sometimes saved work (albeit small) when imported does not work any longer

I assume I have missed something, any pointers to what/how/why please