It cpuld also just be general tolerance to the meth/stimulants as well.
Although there is a degree of cross tolerance between meth and dex it is not exactly 1:1. Ongoing Dexamphetamine use, even at pharmacological doses, is known to increase dopamine release but at the same time chronically reduce dopamine reuptake and metabolisation.
Amphetamine actions include dopamine and norepinephrine transporter inhibition, vesicular monoamine transporter 2 (VMAT-2) inhibition, and monoamine oxidase activity inhibition. However the operation of dex and the operation of meth vary somewhat in exactly what region of the brain they have the most impact.
Regular treatment with dex may cause ongoing reductions in striatal DA, its major metabolite dihydroxyphenylacetic acid (DOPAC), the rate-limiting enzyme tyrosine hydroxylase, the membrane transporter (DAT) and also in the vesicular monoamine transporter (VMAT2).
All of this interacts with the mechanism of meth but meth also interacts more readily with other receptor types (like sigma receptors) and is also far more serotonergic than dex.