Only one of multiple threads is writing - c

Question

I have a task to make a StarCraft like program with multiple pthreads as workers.
So , I have multiple pthreads that run the following function:
void* scv(int num){
int minerals_carried = 0;
while(map_minerals_remaining>0){
minerals_carried = 0;
for(int i = 0; i<number_of_fields; i++){
if(fields[i].minerals != 0 && minerals_carried == 0){
if(pthread_mutex_trylock(&fields[i].mutex)==0){
sleep(1);
// mine returns int
minerals_carried = mine(&fields[i]);
printf("SCV%d is carrying %d minerals from field %d\n",num,minerals_carried,i);
if(pthread_mutex_unlock(&fields[i].mutex)!=0){
perror("pthread_mutex_unlock");
return NULL;
}
}else{
perror("pthread_mutex_trylock");
return NULL;
}
}
}
}
return NULL;
}
I create 5 pthreads and they all get created properly , but only the first one prints out like its supposed to do, and all the other ones dont seem to do anything. Any idea why that might be ?
EDIT :
I was asked to show how I initialized number_of_fields and fields and this is it:
I first declare them as global
typedef struct Mineral_Field_t{
pthread_mutex_t mutex;
int minerals;
}Mineral_Field;
Mineral_Field* fields;
int number_of_fields = 2;
And then I have the following piece of code at the start of the main function:
if(argv[1] != NULL){
number_of_fields = atoi(argv[1]);
}
fields = malloc(number_of_fields*sizeof(Mineral_Field));