Teachers in Denver recently voted to authorize a strike. However, their employer contacted the Colorado Department of Labor and Employment for “intervention” which means that it could take up to 180 days for teachers to legally be allowed to participate in the strike that was authorized by the union. Despite not being allowed to strike, teachers are protesting in order to make their discontent known. The collective activity is occurring as a result of low wages and many years of unsuccessful bargaining.
See "Denver teachers strike won’t begin Monday — and likely not next week, union official says", Elizabeth Hernandez, The Denver Post, January 25, 2019