My thoughts are that maybe you are transplanting them into too large a pot straight away? They need to move up gradually until well established.

Hard to say without more info what the problem is - as Andrea says, too large a pot is possible, too much water, not enough air flow, too cold a temperature, wide temperature fluctuation particularly early on, not enough daylight, poor drainage, all these are possible.

Plant all types of begonias outside in a partly or lightly shaded location after all danger of frost has passed. Place begonias where they get early morning or late afternoon sun to ensure that you get the best bloom. You can also place them in a location where they get dappled sunlight all day. They cannot take prolonged periods of direct sun.

Begonias like fertile soil that is rich in nutrients. You can create this sort of soil by adding peat moss to the area, as well as thoroughly rotted compost. You also have to make sure that your plants’ roots have good drainage or else they could rot. They are excellent companion plants for hostas.

Thank you All for the answers, I think I have committed all the described faults. I think that I have the only plants with web feet (too much water). Tin Tin