invoice not going for second level approval

i have an issue , gurus

there is a invoice which should go through two level of approvals we have built a customised workflow for that , at the fist level there are say 5 approvers who can approve then at the second level there are again 3 , any one among them can approve the invoice at any level , and thereafter he work item gets deleted from the the remaing agents' inbox , now th issue is that after one among the first level approvers approves , it again comes back to him for approval without going to the next level .

what can be the problem and what is the solution , if any one can help me ..