It's important to check your sheaves. You have either wire sheaves which only will work for wire or a line of similar diameter, or a wire sheave shaped like a vee which will acommodate the wire and a line of a larger diameter. Or a wire/rope sheave which is shaped like a U to accomodate the rope with a smaller u in the bottom to accomodate the wire. The third sheave is fine for conversion to all rope while the first and second type are not.
The diameter of the first is obviously too small and the proper size line will not fit. The second type, while large enough to accept a larger size line, will, under load, damage the line.
If you have small wire sheaves, you will probably have a thicker divider between the sheaves. You may be able to replace the divider and then use wider sheaves. Good Luck.