Karin,
Very interesting that you should raise this question. Part of New Years eve after our grandchildren left was spent leafing through some art materials and wondering to myself why I had not made any attempt since art school to do much still life painting. Besides portrait in oil, watercolor and pastel, I have painted watercolor landscapes and bouquets of flowers but no true still lifes. Southwest Art did a series on Still Life and I am determined to outline some projects. I also wondered why I have not painted landscapes in oil and pledged also to do this in 2002.
There may have been a few impressionists who did all three but I will have to do some homework to find that this is so.
My favorite portrait and landcape combo painter is Sargent but I don't recall any still life's by him. I have also been impressed by Sir William Russell Flint who did wonderful figures and landscapes in watercolor but I could not find a still life by him either.
The artist that quickly cames to mind when I saw your post was Richard Schmid who was also part of my reading list on that eve and is equally skilled at figure, portrait, still life and landscape painting. The landscapes include harbors, city scenes, country, woods, etc. in different seasons, weather, and moods.
Early in my art training I found oil to be the medium that I had the most affinity for and relied on it for portrait and figure painting development. At my first job as a greeting card artist I had to develop my skills as a watercolorist and eventually did fairly well at florals and landscapes in this media. Perhaps this is a circuitous route to saying that you might want to try doing landscapes in another medium. The break from subject and media could help you get past whatever it is that blocks your progress.
I have often wondered if the thinking process, technique, and directness of watercolor as might be found in Homer and Sargent paintings enhanced their skill at oil painting.