You should write to your provider and ask them to respond to your complaint.
If your complaint is against the trustees or managers of an occupational pension scheme, you need to ask them to deal with your complaint under the scheme’s internal dispute resolution procedure (IDRP).
The scheme’s administrator should be able to give you details of the IDRP.
The ombudsman only deals with complaints about occupational and personal pension schemes. It doesn't deal with complaints relating to state pensions.
For complaints relating to state pensions it is best to approach TPAS.