That's quite an extreme "behalf" for the violets but perhaps are they in a too much shady place? They also need low temperature to bloom. This added maybe with too much nitrogen could bring violets to make only leaves.
It would be interesting to notice wether flowers appear later. I suspect them to have a determined quantity of flowers to produce and if they don't appear early,they would appear later...Let us know please!( I have been able to harvest a small bunch on may 3 years ago!)
Cheers
Nathalie

Chère Nathalie,
thank you very much. The spot where my violettes grow, gets about 1 1/2 hours of morning sun at the moment. It was very warm in the last two weeks in Southern Germany, may be that's a reason. And of course the violettes are surrounded bys other plants, e.g. rhododendron that get of course guite a bit of fertilizer. I'll let you know, if the flowers appear later in the year.
Michael