Onions

Onions.  I have grown onions for several years and never really thought very much about it.  They always seemed to do fine and store perfectly.  Suddenly the past two years they have been misbehaving.  They don't seem to lodge, begin to turn brown, and mature at the end of the season.  Instead, it seems as though they begin to soften and turn a little green in the center as through they were getting ready to go to seed.  It's the same variety I've always grown but I think next year I need to try another kind and do some study about growing onions!  What would my grandmother say???