The height of the radio-reflective layer of the Earth's ionosphere changes according to the time of day. (It's higher at night). Additionally radiation directly from the sun (which can block reception of radio & TV signals) is only present during the day. Those factors affect the reception of TV signals by your aerial. (A lot of other things can also affect the strength of signals arriving at your aerial. They include weather conditions, atmospheric pressure, the 11-year sunspot cycle and individual solar flares). If your aerial isn't really up to the job (which it clearly isn't) it will only provide good enough signals at certain times.
Further, different brands of TVs, Freeview boxes and DVD recorders have different 'sensitivities' (i.e. the ability to 'pull in' weak signals). Your DVD recorder has got better sensitivity than your other devices, so it can cope better with weak signals.
You might need to replace your aerial. (If so, replace the cabling as well. Modern cables, designed specifically for UHF TV signals, have lower signal losses than older cables). However a cheap signal booster between the aerial and the splitter unit might fix the problem.
Chris