Did you try tracing an edge segment on the spline? If so and there’s no face formed, either there’s a gap in the spline or it isn’tall in the same plane. Share the .skp file so we can see what you actually have going on.
thank you 
may i know how do you trace the edge and how do i fix it ?
does redoing it, is the only way
wow thanks,
how do you find the gap ? i tried zooming in along line still nowhere to find
When you do the kind of fixing that Dave did, you draw lines across the gap, and the side that fills in is ok, and the side that doesn’t fill in has a gap somewhere. A few lines later you know the small area to zoom into to look for problems.
If you don’t look for the gap, eventually the line you draw fills in both sides, which it does by effectively repairing the small gap. So, you could draw the lines, erase them after everything is filled in, and never worry about the small gap that you had fixed.
ok thanks really helpful