Call stored procedure in sap Hana

I have some layouts that are designed using a stored procedure in SAP Hana.  The run for the majority of layouts however on some i get the message 'Failed to retrieve data from the database'-

if i call the stored procedure in Hana with the posting in question i.e. docentry param and object id i get the following message:

Call "iis_epc_eml_mdoc" (739,17)

Could not execute 'CALL "NCC_LIVE"."IIS_EPC_EML_MDOC_NCC_SO" (539,17)' in 222 ms 602 µs .

SAP DBTech JDBC: [7]: feature not supported:  [7] "NCC_LIVE"."IIS_EPC_EML_MDOC_NCC_SO": line 431 col 4 (at pos 20819): [7] (range 3): feature not supported: invalid unicode string (support only CESU-8) at function __cs_field_HybridNCLob__()

The error seems to be intermittent i.e. it will work on a posted document and then stop working or vice versa - a document wont work and then will start working again!!

any help greatly appreciated!!