What happens with mine, which has just had software ver.1.2.0 installed (it was misbehaving before with 1.1.0 in) is this the unit boots up and does not show the "C" or "T" icons at all or may show only a few. If you exit the active nav page and cursor away to another location then back to where you know the were the "T" & "C" icons they have gone.
Lowrance's work around which does work is to select the "Miami Demo" map option then reselect "Lowrance" in the map options menu this puts back all the missing icons, but you need to remember to do this every time the unit is switched on.
They did not admit to seeing other units with this fault, it isnt the chart card or the card reader they believe, so software appears the most likely candidate.
New boat is here, very happy!