One problem with many current approaches to quantum gravity, including string theory is, that these models are extremely flexible, involving lots of empirically unknown, freely tunable parameters.
As a result, most tests in this domain merely rule out parts of the parameter space, rather than properly ruling out any one of these theories as a whole.
If such a test finds new results that rule out the currently most popular variants of a given theory, usually all that means is, that people tweak the parameters and find a new variation that works after all.
Critics of many of these theories have compared this to Epicycle theory which is an outdated model of planetary motion wherein, at first, all planetary orbits were assumed to be circular, with the Earth as stationary in the center, and in order to correct theoretical orbits to better fit observations, further circles were simply introduced - planets would then orbit on circles which themselves orbit on circles.
This theory was later rejected by a much simpler one that puts the sun at the center of the solar system, and allows for more flexible conic-section-based orbits, able to explain the same observations in much simpler equations.
There was a hope, that the Large Hadron Collider (LHC) in CERN would result in new kinds of particles besides the Higgs boson. Speculative particles predicted by Super Symmetry (SUSY) theories where every regular particle has a super symmetric partner with different kinds of properties. This has not yet happened, narrowing out the simplest versions of these theories. To further test these theories, more powerful colliders might be needed, to probe ever higher energies.
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