The begonias you buy as plug plants are fibrous rooted begonias, a different type to the ones that grow from tubers, though occasionally the fibrous rooted ones can make little tubers, I'm told.
Sometimes the tubers you buy are dried out or they can rot. The one in the pic has got some buds showing so keep it and wait would be my advice. What sort of temperature have you got it in? We've had a cold spell for the time of year that's making all sorts of normally reliable plants late.

This tuber looks a bit wet ...it would have been better planted in multi purpose compost and the tuber should be covered with compost too. They need to be kept on the warm side....

I was following the advice of the Thompson and Morgan leaflet which can with the tubers. It said to plant in John Innes compost with some sand mixed into it, then water from underneath so as to prevent the indent on the upperside having water sitting in it. My thinking is that if I bury it, it will be all the more likely to have damppness in the crown and start rot. I suppose I could cover them and do as they said,- water from beneath, not below. It is very well drained.

My begonia corms that i bought from T&M are three years old. I use multi purpose compost and sit the corms half way deep. They have to be kept in a warm place and kept just moist to avoid rotting. It does take time especially this year. It's so exciting to see the tiny buds coming through and then growing into leaves. Your corm looks very wet so you should hold off the watering and let it dry a little.Good luck.