Got off Lexapro then had to reintroduce. Now it's not working.What happened?

"SRIs and SSRIs. The SRIs and SSRIs block serotonin reuptake, increasing the amount of synaptic serotonin; this increase in available serotonin leads to a compensatory decrease in the number of some postsynaptic serotonin receptors (down-regulation).17 When the SSRI is discontinued abruptly, reuptake is reestablished (and possibly enhanced transiently), lowering the amount of available serotonin within the synapse. The resulting reduced concentration of available serotonin, plus the receptor down-regulation, causes clinical signs of serotonin deficiency. Since serotonin interacts with numerous other neurotransmitter systems, its deficiency has the potential to disrupt many other neuronal functions.
SSRI discontinuation symptoms are similar to those of the TCAs, with dizziness, gastrointestinal symptoms, and sleep disorders common. Anecdotal reports have included complaints of “electric shock–like” sensations, flashes, and “withdrawal buzz.”15,17,18,23–26,28–32 The type and severity of the symptoms correlate with the relative affinities of the agents for the serotonin reuptake sites and with secondary effects on other neurotransmitters; with SRIs that also affect cholinergic systems, the symptoms possibly correlate with cholinergic rebound.

Sudden discontinuation is a particular problem with agents that have a relatively short half-life, which include the SSRI paroxetine and the SNRI venlafaxine. With these types of drug, the symptoms can last up to 2 weeks.29 A randomized clinical trial reported that in the 5 to 8 days after discontinuation of an SSRI, symptoms were most common with paroxetine—the most potent SSRI and one with a short half-life—and least common with fluoxetine—an SSRI with a lower serotonin reuptake affinity and a longer half-life.31 Reactions to sertraline were mild and infrequent. However, this trial was designed to simulate intermittent noncompliance and did not attempt to document late-emerging effects that might occur with a drug with a long half-life, such as fluoxetine. There have been reports of symptoms starting 25 days after fluoxetine withdrawal with some persisting as long as 56 days.29 Late reactions to fluoxetine cessation may be reported less often because trials do not follow patients long enough after withdrawal to observe them. Further, the symptoms may be mild and patients may not attribute general symptoms such as flu-like complaints, dizziness, or sleep disturbances to a drug they had stopped taking weeks earlier.29

In addition to its effects on serotonin reuptake, paroxetine affects muscarinic receptors, and its withdrawal could contribute to symptoms of cholinergic rebound, a mechanism usually not attributed to other SSRIs.23 One case report of a patient discontinuing low-dose paroxetine (10 to 20 mg/day) noted symptoms that were very disabling initially and lasted longer than the 5 to 15 days mentioned in the prescribing information. The responses included agitation, anorexia, nausea and diarrhea, vertigo and dizziness, paresthesia, and a shaking chill.30 Sertraline, which like paroxetine is associated with a higher occurrence of discontinuation reactions, is the only SSRI that strongly blocks dopamine reuptake as well as serotonin-reuptake sites.23 Further, discontinuation symptoms with sertraline, although usually mild, may be related to its effect on both dopamine and serotonin reuptake."

Source: National Library of Medicine

It would be a good option after all the time you have restarted Lexapro and as you mention it is not doing its job to make a joint decision with your Psychiatrist to try a different SSRIs.