All the following studies were done using patients with depression and not anxiety but I would assume the numbers are correct. Drs usually recommend an snri (like cymbalta) if the patients ssri poops out on them as the snri are less likely to.
Tachyphylaxis
Tachyphylaxis, also known as drug tolerane or"Prozac poop-out, is the condition where a patient becomes tolerant of a drug and its effectiveness declines.
https://www.ncbi.nlm...les/PMC4008298/
"...depressed patients with “true” antidepressant tachyphylaxis may be less responsive to new treatment interventions."
https://www.ncbi.nlm...pubmed/19571597
"The odds ratio indicates a 19.9% reduced likelihood of response with each prior antidepressant treatment" In otherwords for each antidepressant you have been on there is a 19.9% greater chance that the next AD will poop-out on you.
https://www.ncbi.nlm...pubmed/21851459
The cases of tolerance/tachyphylaxis were observed in 23% of patients and in 13 trials (22.4% of trials). All cases of tolerance occurred during monotherapy. No cases of tachyphylaxis were observed in the non-SSRI group (SNRI) while in the SSRI group, tolerance at some stage of the treatment was detected in 41.9%.
https://www.ncbi.nlm...pubmed/15960562
Rates of tachyphylaxis were significantly lower with the dual reuptake inhibitors venlafaxine and TCAs (3.7% of 82) compared to rates of tachyphylaxis with SSRIs (14.1% of 326).
https://www.ncbi.nlm...pubmed/18694599
The odds of responding to venlafaxine therapy decreased with an increasing number of prior antidepressant exposures.