ben: I thought for a while that that was Pshat in the Passuk. But I think the Esnachta splits it up differently. It means “because you did not worship Hashem your God with happiness and goodness of heart, while you had plenty.” The Esnachta splits up the sentence with B’simcha Uvtuv Levav going back on the Asher Avadta, with the Rov Kol defining why we should have had no issue being happy serving Hashem and why it’s a punishable offense (implying that it’s not as bad to serve Hashem unhappily in bad times, which makes sense).
Also, if all three phrases (Simcha, Tuv Levav, Rov Kol) were referring to the conditions in which we should have served Hashem, then they would have the same prefix. They don’t. The Mem before Rov Kol tells me that my reading of how to break up the Passuk is correct.