Hello there Martin 🙂
In my sxperience this could possibly be either of the following issues ,
The size of the podcast can sometimes be too big even though the size seems reasonable it can be an issue for the mobile data * This has happened to me *
The website you are downloading from may have compatability issues when downloading.
As you have no issue via wifi id ayre on the side ot number 1.
**** Did some more research **
If you have an Ios based phone * Apple * it appears they have a setting that prevents you downloading over 100mb items via mobile data despite available data. THIs appears to be on all networks.