My sketch is fully defined but when i change a global viralble to scale up the sketch, the sketch become unsolvable by solidworks and i get a warning to correct the sketch.
3 Answers

From my experience Solid deals really bad with this kind of sketches. The other day I set up a complete machine housing using 3D sketches which operated with global variables. Had the same problems when I exeeded a point of too much dimensioning in one sketch. I would recommend splitting the sketch into several ones. I like your idea of using only one sketch for every contour element, I would prefer it in the same way, but that always caused equation resolving issues. I don’t know exactly why this error comes up, but I think it is caused by the order of resolving the equations. Like if you edit a dimension causes a zero length of a sketch entity. If a necessarily needed "pre-equation", whoch might prevent that event, is calculated after the current one, the error occurs and the resolving will be stopped.

First off, keep your sketches simple. Instead of everything being in one sketch and feature. Break it down into multiple steps.
Second, most dimensions are actually ambiguous. Meaning the geometry may be locked into a specific distance but relative positions can swap and the dimension still be valid. Depending on the order you placed dimensions and how you have chained them together geometry can swap positions during the scaling action. To maybe prevent this from happening try increasing the scale just a small amount at a time and creep up on the final scale. You will have to experiment to see how small a scale increase you must use.
I have done the exact same thing you’re attempting and had the same issue. Slowly increasing the scale factor solved the problem.

Solidworks deals the sketches (lines, conic sections etc.) with equations. In a plane, these are algebraic manipulation of coordinate geometry with two axis. Same goes with 3D sketches where axis are three. Solidworks also maintain a relation among the lines and conic sections like parallel and perpendicular lines, vertical lines, horizontal lines, distances between various points, tangents etc. If a sketch is drawn using a global variables and if any one of the relations become invalid after changing the global variable, Solidworks generates error in the sketch. Display/delete relations will show all the relations a sketch has and shows which relations are not valid. Deleting these unresolved relations one by one will make the sketch error free keeping all the dimensions intact. After doing that, relations can be added to make the sketch more definable from all respect.