ALL collinears are very difficult for DIY. Reason: being multi element, require phasing elements between vertical receiving elements.
The phasing elements are extremely sensetive to dimensions. An error of few mm in DESIGN, or CONSTRUTION can push a collinear from excellent to poor category. The phasing element are coils (coiled whip), stubs (franklin), core wire of coax (coco).
The collinears are best when they are designed by Computer Aided Design software, followed by production using templates and precision metal forming macinary, followed by testing/trimming/adjustment with test equipment & anechoic chamber.
It seems the designer of your collinear has paid no attention to phasing coil's accuracy. The design does not include height of the coil, a very important dimension fir correct phasing.