New Post: labels disappeared

First I added labels for the shapefile, I didn't set the scale options of labels.
I zoom out the map, the labels are displayed well. Then I zoom in the map , the labels are disappeared. After this, I move the map somewhere, the labels are appeared.

My code is here:

sf.Labels.AutoOffset = false;
map.set_LayerLabelsScale(labellayerindex, false);
map.set_ShapeLayerPointSize(labellayerindex, 14);
map.set_ShapeLayerPointColor(labellayerindex, (uint)ColorTranslator.ToOle(Color.Brown));
map.set_ShapeLayerLineColor(labellayerindex, (uint)ColorTranslator.ToOle(Color.Lime));
map.LabelColor(labellayerindex, (uint)ColorTranslator.ToOle(Color.Orange));
sf.Labels.Generate(expression, tkLabelPositioning.lpCenter, false);