You didn't mention what version of mojoPortal nor what version of Artisteer so I'm only guessing you are using the latest of both.
I would compare how your <portal:SiteMenu is configured in layout.master vs the Artisteer skins we ship in the /Data/skins folder
They are all declared like this:
<portal:SiteMenu ID="SiteMenu1" runat="server" UseTreeView="true" TreeViewPopulateOnDemand="false"
HideMenuOnSiteMap="false" SuppressImages="true"></portal:SiteMenu>
Make sure yours isn't using some different configuration.
Also compare the treeview settings in your theme.skin file should be like this:
<portal:mojoTreeView runat="server" SkinID="SiteMenu"
ContainerCssClass=""
RootUlCssClass="art-hmenu"
RenderLiCssClasses="true"
RenderAnchorCss="true"
LiCssClass=""
LiRootExpandableCssClass=""
LiRootNonExpandableCssClass=""
LiNonRootExpnadableCssClass=""
LiSelectedCssClass="active"
LiChildSelectedCssClass="active"
LiParentSelectedCssClass=""
AnchorCssClass=""
AnchorSelectedCssClass="active"
ExpandDepth="-1"
ShowExpandCollapse="false"
PopulateNodesFromClient="false"
SuppressCornerDivs="true"
MaxDataBindDepth="-1"
/>
<portal:mojoTreeView runat="server" SkinID="PageMenu"
ContainerCssClass="art-box art-vmenublock"
ExtraMarkupMode="Artisteer"
RootUlCssClass="art-vmenu"
RenderLiCssClasses="true"
RenderAnchorCss="true"
LiCssClass=""
LiRootExpandableCssClass=""
LiRootNonExpandableCssClass=""
LiNonRootExpnadableCssClass=""
LiSelectedCssClass="active"
LiChildSelectedCssClass="active"
LiParentSelectedCssClass=""
AnchorCssClass=""
AnchorSelectedCssClass="active"
ExpandDepth="-1"
ShowExpandCollapse="false"
PopulateNodesFromClient="false"
SuppressCornerDivs="true"
/>
Hope that helps,
Joe