Workflow failing when more than one user in the approval sequence

Hi! - wondering if anyone out there can help before I pull my hair out

We have a client using workflows for their order approvals, approval user setup is all setup and working well on the Sales side, however on the Purchase side - invoices and credits seem to get 'stuck' when the approval sequence is more than one and the record gets locked on record restriction and currently we have to manually delete the record restriction to get around it - it's all out of the box stuff and have tried rebuilding adding in lines to ensure the restriction is removed.

Please can anyone offer any help, I really would be most grateful.

Thanks,