Given that Ceefax closed down over 5 years ago, with the ending of analogue TV in the UK, I wouldn't expect you to be able to view it!
However I assume that you're referring to the 'red button' text information services provided by the BBC. They're often the first things to go when there's insufficient signal strength. (The strength of the signals arriving at your aerial doesn't remain constant. They're affected by the 11-year sunspot cycle, individual flares on the sun's surface, atmospheric pressure, seasonal changes in the ionosphere, precipitation, local electrical interference and many other things).
Installing a signal booster (around a tenner is some big supermarkets) should sort the problem out for you.