If you could please post some pictures it would help us to determine what is going on.
Although Chinese Elms are often advertised/promoted as indoor trees I think you will find that most experienced growers would rather see them be kept outside where the light and humidity are more appropriate. Temperature fluctuations and air flow are not to be underestimated as they are also important, all of these play a role in the health of your tree.
I water the tree by spraying it,
I'm not entirely sure what you mean by this but your tree should be saturated not simply sprayed/misted. Sorry if I misinterpreted your meaning. Refer to 'the chopstick method' as outlined here.
I have even re potted it
Hopefully you used a decent bonsai soil that has texture and drains freely. Conventional potting soil is not appropriate. More information would also be helpful here. How long had you owned it before re-potting and how long ago was that. Where does the re-potting fit into the big picture? I assume you re-potted it after it began to decline.
it has never been kept in the light for too late and neither has it been kept in the cold.
The tree needs good light to thrive and this species is able to withstand more cold than it seems you are aware.