See this article, especially the red-yellow-green chart in Figure 1 which shows which drugs (Gleevec, Sprycel, and Tasigna only) are foiled by certain kinase mutations:
Q252H is the 4th kinase mutation listed in the chart, and is colored "yellow" for all 3 mainline drugs (Gleevec, Tasigna, and Sprycel). This means these drugs only work to some degree against this type of mutation, so your results on any of them may be marginal.
This color chart includes the TKI drug Bosulif, and shows that it overcomes Q252H:
This article says Ponatinib (Iclusig) can overcome Q252H mutation with 20mg dosage
(See chart number 3)
The maker of Iclusig says that it "overcomes all clinical mutations tested".
"ponatinib is active against many of the other imatinib-resistant mutations, including M244V, G250E, Q252H, Y253F/H, E255K/V, F317L, M351T, and F359V, among others"
So it would be a very good idea to switch to Iclusig (Ponatinib) or Bosulif. One of these should turn this around for you. But you need to do this as soon as possible to avoid further loss of response.