When the generator is left unused, Mo decays to Tc, which in turn decays to Tc. The half-life of Tc is far longer than its metastable isomer, so the ratio of Tc to Tc increases over time. Both isomers are carried out by the elution process and react equally well with the ligand, but the Tc is an impurity useless to imaging (and cannot be separated).
The generator is washed of Tc and Tc at the end of the manufacturing process of the generator, but the ratio of Tc to Tc then builds up again during transport or any other period when the generator is left unused. The first few elutions will have reduced effectiveness because of this high ratio.