Offset lnb only suitable for that type dish. An offset dish is really, in simple terms, only part/segment/portion of that total dish you have. The lnb only "sees" that smaller reduced area, and sees the rest as "noise".
Your prime parabola is designed for the whole dish you have, focussing the whole dish area into your prime feed.
You need to buy a prime feed, possibly even a combined C and KU band LNBF all in one, may be better for you, so you can use your dish for both bands.
Hope this helps......
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