This will depend on what type of chart you have.

You can try chart.axis.x.tickmarkintervaltype property.  If you set it to smart it will out where to place the labels by itself.  If you set it to percentage, it will use the tickmarkpercentage property.  If you set it to datainterval, it will use the tickmarkinterval and tickmarkintervaltype properties.

If that doesn't work, try setting chart.axis.x.labels.layout.behavior to none.  If you see your labels then, you can probably use this Knowledge Base article to resolve your issue:

http://devcenter.infragistics.com/Support/KnowledgeBaseArticle.aspx?ArticleID=4228