MDS WebService - adding Number or DateTime MetadataAttribute

Question

I am currently using the WebService to dynamically build a model in MDS (SQL Server 2008 R2).  All seems to be working fine with one exception so far - when I add a Number or DateTime attribute to an entity, it (and all attributes I added after it) does not appear in the Leaf Attributes section of the Edit Entity screen in System Administration until I go in and edit an attribute that does exist and simply save it.  I am thinking that maybe I am missing something in my code when I add the new MetadataAttribute.  Here is an excerpt of my code:

            public MDS.OperationResult AddAttribute(MDS.Identifier modelId, MDS.Identifier versionId, MDS.Identifier entityId, string attributeName, MDS.AttributeType attType, MDS.AttributeDataType attDataType, int dataTypeInfo, string mask, MDS.Identifier dbaEntityId)
            {
                MDS.OperationResult or = null;
                MDS.Metadata md = new MDS.Metadata();
                or = new MDS.OperationResult();
                md = GetMetaData(new MDS.International(), ref or, modelId, versionId, entityId, MDAction.AttributesOnly);
                MDS.MetadataAttribute ma = null;

                if (attType == MDS.AttributeType.FreeForm)
                {
                    if (attDataType != MDS.AttributeDataType.DateTime)
                    {
                        MDS.Identifier maskID = null;
                        if (!string.IsNullOrEmpty(mask))
                        {
                            maskID = new MDS.Identifier() { Name = mask };
                        }

                        ma = new MDS.MetadataAttribute()
                        {
                            AttributeType = attType,
                            Identifier = new MDS.MemberTypeContextIdentifier()
                            {
                                EntityId = entityId,
                                ModelId = modelId,
                                MemberType = MDS.MemberType.Leaf,
                                Name = attributeName
                            },
                            DataType = attDataType,
                            DataTypeInformation = dataTypeInfo,
                            InputMaskId = maskID,
                            DisplayWidth = 100
                        };
                    }
                    else
                    {
                        MDS.Identifier maskID = null;
                        if (!string.IsNullOrEmpty(mask))
                        {
                            maskID = new MDS.Identifier() { Name = mask };
                        }

                        ma = new MDS.MetadataAttribute()
                        {
                            AttributeType = attType,
                            Identifier = new MDS.MemberTypeContextIdentifier()
                            {
                                EntityId = entityId,
                                ModelId = modelId,
                                MemberType = MDS.MemberType.Leaf,
                                Name = attributeName
                            },
                            DataType = attDataType,
                            InputMaskId = maskID,
                            DisplayWidth = 100
                        };
                    }
                }

                List<MDS.MetadataAttribute> lst = md.Attributes.ToList<MDS.MetadataAttribute>();
                lst.Add(ma);
                md.Attributes = lst.ToArray();

                MDS.ServiceClient proxy = CreateMdsProxy();
                proxy.MetadataCreate(International, md, true, out or);

                return or;
            }

Any feedback would be appreciated. Thanks!