Workflow Erasing Contents / Changing a Field Value

Hi

I have created a workflow that assigns a Membership # to the Member record.
So say I create a New Member record, add in my data then click Save & Close.
When I re-open the Member record, the Coverage Type has changed to the default value of E-Single (1st one in list)
and the Parent Company field has been erase.

Is there anyway I can fix this?

Thanks.