Great question! It is very unusual to have urine retention secondary to the urethral sphincter not relaxing (a lack of coordination between the bladder contracting and the urethra sphincter relaxing). Usually the cause is secondary to a neurogenic etiology such as multiple sclerosis. Botox injections of the external sphincter muscle have been used to relax the external sphincter. The causes of urine retention include a weak bladder muscle that is not able to contract efficiently or an external sphincter muscle that won't relax. It may also include obstructive causes in women such as pelvic organ prolapse, inability to relax the pelvic floor muscles, and rarely, a urethral stricture. Depending on the suspected cause and degree of urine retention, treatments will vary from hands on physical therapy of the pelvic floor, medication, botox, intermittent self-catheterization, indwelling foley catheter, urinary diversion, and an InFlow device to resolve urine retention. I recommend you follow up with a urologist/urogynecologist that is well trained in female urology. Hope you find a resolution!