In the case of CBeebies/CBBC, the SD versions were removed quite a while ago and became HD-only. The hidden-EPG data that allows "old" recordings from the (now-removed SD) channels to still work, was retained for a while and was removed around March 2020 IIRC. This forum post from 9months ago covers this exact question.
The answer remains the same - the vast majority of the time, there isn't a time limit on recordings being kept. But there are circumstances where there is, and I'm guessing the programmes you're trying to watch were broadcast around a year ago.
You may be able to find these programmes using the iPlayer, or perhaps they'll be shown on CBeebies/CBBC (HD) again at some point.
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