小中大Three possibilities:
1. before you chaged your column with Ni2+, wether you cleaned the column with NaoH? if you did, maybe you did not fully wash away the NaOH and then start charging
2. your equilibrated buffer, you mentioned Pi, does pH ok, I mean not so much NaOH? same solution, keep NaOH (or high pH) away your Ni2+
3. your dissolved solution GudmHCl, contained too much DTT or reduced reagent. dilute you loading solution untill DTT acceptable ny Ni2+