问Ngx Bootstrap元素未显示

EN

import { NgModule } from '@angular/core';
import { TooltipModule } from 'ngx-bootstrap/tooltip';
import { BsDatepickerModule } from 'ngx-bootstrap/datepicker';

@NgModule({
  imports: [
    BsDatepickerModule.forRoot(),
    TooltipModule.forRoot()
  ]
})
export class UiElementsModule { }

import { AppComponent } from './app.component';
import { BrowserAnimationsModule } from '@angular/platform-browser/animations';
import { LayoutsModule } from './layouts/layouts.module';

@NgModule({
  declarations: [
    AppComponent
  ],
  imports: [
    BrowserModule,
    AppRoutingModule,
    BrowserAnimationsModule,
    LayoutsModule,
    UiElementsModule
  ],
  providers: [],
  bootstrap: [AppComponent]
})
export class AppModule { }

<button type="button" class="btn btn-primary" tooltip="Vivamus sagittis lacus vel augue laoreet rutrum faucibus.">
    Simple demo
</button>