If the protein comes out at low imidazole concentrations, it is binding weakly to the metal ions. Run an imidazole gradient, most of the times a single chromatography step is not enough to get pure proteins, even with a his-tag.

I have not done single chromatography ... i have already run immidazole gradient for washing from 20 to 50 mM but at low all the protein with cas12a came out in elution but when i increase from the concentratio of cas12a in sds also decreses alongwith the other proteins and at higher the protein is washed out at washing step

You do not provide enough details, what are the adsorption conditions? what are the washing conditions? SDS? Gradient from 0 to 20 mM first?

ecoli BL21+ strain was used for protein expression with 0.5mM IPTG concentartion at 18 degree C overnight. lysis buffer (500mM NaCl, 20mM hepes buffer and protease inhibitor tablet) sonication for 5 mins (5 sec on and 10 seconds off) centrifugation at 20,000 rpm for 40 mins at 4 degree C. for binding buffer (500mM NaCl, 20 mM hepes, 5-10mM immidazole pH7.5 and 8) washing buffer (500mM NaCl, 20 mM hepes, 20-50mM immidazole pH7.5 and 8) anad elution (500mM NaCl, 20 mM hepes, 200mM -500 immidazole pH7.5 and 8).

As I said before, even with the tag, the protein could be folded in such a way that the interaction with the nickel ions is not strong enough. If the protein becomes adsorbed at 5-10 mM imidazole and desorbs in 20-50 mM imidazole with washing, wash with the binding buffer, afterward, run an imidazole gradient (very slow) to 50 mM imidazole (or less).