Answer: If these are first year plants, they may not be mature enough to bloom yet. Lavender is nearly shrub-like and usually takes a year or so to settle in, put on some size and maturity and begin to bloom. Sometimes a new plant will bloom for the first time late in the summer. Otherwise, expect blooms in June and a rebloom later on in the season. (Lavender does not bloom continuously.)
Lavender requires full sun to bloom; in a shaded or part shade location it will not bloom much if at all.
As far as the soil, lavender must have a well drained spot, but it does not require a rich soil. Sand would improve drainage if drainage is a problem but would not improve fertility. The drainage is most important in terms of winter survival -- in a poorly drained location lavender usually dies off during the winter. However, replanting it would possibly set it back. August is the worst time of year for transplanting or resetting plants. If you want to try this or need to move it to a sunnier spot, wait until the end of the month.
In any case, I would not expect any changes made this year to enable it to bloom this year. I hope this helps you trouble shoot.
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