C :: Point Operations On Elliptic Curve In A Prime Field

I am trying to write a program to perform point operations on a elliptic curve in a prime field I am using the standard formulaes for point additions and doubling in my code and these operations are performed by functions that are called but I am getting output for certain points but not all.

Code:

structure point_multiply(int x, int y, int k ) {
int xk;
int yk,m;
xk=x;
yk=y;
m=1;
int xL,yL,s,e;

[code].....

s1, s2, s3 etc are structures which hold a 2 integers which act as x and y co-ordinates.I am getting output by entering k=3,g=4, h=5 and many other cases mostly with small numbers but not for larger numbers.