A button actually labelled 'Text' would normally only be there to access the old analogue service which, as Vicasso indicates, is no longer available.
'Red button' text services (which is what I suspect you're actually referring to) require a stronger TV signal than normal TV reception does. The signal strength received at your aerial doesn't remain constant. It's affected by the 11-year sunspot cycle, individual solar flares, atmospheric pressure, precipitation, interference from other electrical devices in your home (such as computers, routers, TV sets, etc, etc) and similar interference from your neighbours' houses, as well as many other factors. You're obviously getting sufficient signal strength to access the red button text services at certain times of day but not at others. Your aerial will also be better at receiving some multiplexes (= 'channel bundles') than others, which is why you're sometimes able to access text via Channel 80 but not via Channels 1 or 2.
Try going directly to BBC Red Button on Channel 200 and/or improving your signal strength with one of these:
http://www.maplin.co.uk/p/1-way-aerial-amplifier-rt67x