The Mostly Mozart Festival Orchestra is greatly improved since the appointment of music director Louis Langrée. But it reached a plateau a few seasons ago and has not made any significant strides forward since. Tuesday evening at Avery Fisher Hall provided an illustrative case in point.

Admittedly the group had little to do during the first half of the program. The guest conductor, Jiri Belohlávek, brought some music from his homeland, the Serenade No. 2 of the Czech composer Bohuslav Martinu. A solid craftsman who spent his later years in America, Martinu was not blessed with a significant talent for melodic invention, leaving much of his work clever but not memorable. This little piece, for 16 high strings — no cellos or double basses at all — was based on folk tunes and so flowed a little more pleasantly than some of the movements of his six symphonies, but was little more than a daisy chain of sugary ditties.

It is gratifying when a young aspirant makes a strong impression upon a critic and then proceeds to blossom into a mature artist. Dutch violinist Janine Jansen made her “distinctive debut” at Weill Recital Hall in 2000 and I predicted great things for her at the time. Today she is at the top of her game and she performed expertly in Mozart’s Sinfonia Concertante in E-flat major, K. 364.

Joined by the Ukrainian violist Maxim Rysanov, Ms. Jansen filled the cavernous hall with her bright tone, especially impressive in the sotto voce range. This was full-bodied Mozart playing — emotional phrasing, dramatic gesture, genuine feeling. The brilliant sound of Ms. Jansen’s 1727 Stradivarius, known as “Barrere,” blended deliciously with Mr. Rysanov’s 1780 Guadagnini viola. The orchestra was held firmly in check and provided a balanced and unobtrusive background.

After intermission, the group had its opportunity to shine, but simply did not. Instead we heard a run-of-the-mill Symphony No. 7 of Beethoven that was long on perspiration but short on inspiration. The ensemble lacked the heft to pull off the first movement, with gravitas replaced by good intentions and a bit of a singsong feel. This was a tasteful performance, but that is not the ideal for Beethoven. There was a palpable lack of energy, no rough-and-tumble quality, no rebellion nor recalcitrance.

The Allegretto was played too fast, relegating some of the most profound expressions of anguish in the history of art into simple kvetching. Even if you are in the camp that blesses this type of hurried approach, much of this evening’s phrasing was tentative, almost cartoonish. And the finale, that great, swirling dance, was a flaccid flop, accents and emphases missing in inaction. What should have been an exciting ending was simply a foregone and merciful conclusion.

Perhaps this type of pedestrian interpretation results from a shortage of rehearsal time. Unlike, say, the Philharmonic, which changes programs once a week, this orchestra is expected to communicate French music under Mr. Langrée one day and then just two days later plumb the depths of the Teutonic with mature Beethoven. Considering that it does not play as a unit all year, it does a creditable job. But the festival may need to consider whether we would be better off with more variety or higher quality.