如果Angular 2+ Universal页面在服务器上不渲染，可能是由于以下几个原因引起的：

1. 未正确配置服务器端渲染(SSR)。确保已正确设置Angular Universal并在服务器端启动应用程序。以下是一个示例：

// server.ts
import 'zone.js/dist/zone-node';
import 'reflect-metadata';

import { enableProdMode } from '@angular/core';
import { ngExpressEngine } from '@nguniversal/express-engine';
import * as express from 'express';
import { join } from 'path';

import { AppServerModule } from './src/main.server';

enableProdMode();

const app = express();

const PORT = process.env.PORT || 4000;
const DIST_FOLDER = join(process.cwd(), 'dist/browser');

app.engine('html', ngExpressEngine({
  bootstrap: AppServerModule
}));

app.set('view engine', 'html');
app.set('views', DIST_FOLDER);

app.get('*.*', express.static(DIST_FOLDER, {
  maxAge: '1y'
}));

app.get('*', (req, res) => {
  res.render('index', { req });
});

app.listen(PORT, () => {
  console.log(`Node server listening on http://localhost:${PORT}`);
});

2. 未正确配置Angular Universal的AppModule。确保在AppModule中使用BrowserModule.withServerTransition方法来配置服务器端渲染。以下是一个示例：

// app.module.ts
import { BrowserModule } from '@angular/platform-browser';
import { NgModule } from '@angular/core';
import { BrowserAnimationsModule } from '@angular/platform-browser/animations';
import { ServerModule, ServerTransferStateModule } from '@angular/platform-server';
import { HttpClientModule } from '@angular/common/http';
import { AppComponent } from './app.component';
import { AppModule } from './app.module';

@NgModule({
  imports: [
    BrowserModule.withServerTransition({ appId: 'my-app' }),
    BrowserAnimationsModule,
    ServerModule,
    ServerTransferStateModule,
    HttpClientModule,
    AppModule
  ],
  bootstrap: [AppComponent]
})
export class AppServerModule { }

3. 未正确使用Angular的Universal API。确保在组件中使用isPlatformServer和TransferState等API来处理服务器端和客户端之间的数据传递。以下是一个示例：

import { Component, Inject, PLATFORM_ID } from '@angular/core';
import { isPlatformServer, TransferState } from '@angular/platform-browser';

@Component({
  selector: 'app-root',
  template: `
    
{{ title }}
    
{{ message }}
  `
})
export class AppComponent {
  title: string;
  message: string;

  constructor(
    private transferState: TransferState,
    @Inject(PLATFORM_ID) private platformId: Object
  ) {
    this.title = 'Angular Universal';
    this.message = this.transferState.get('message', '');

    if (isPlatformServer(this.platformId)) {
      // 在服务器端设置消息
      this.transferState.set('message', 'This message is rendered on the server');
    }
  }
}

通过确保正确配置服务器端渲染、AppModule和使用Angular Universal的API，您应该能够解决Angular 2+ Universal页面在服务器上不渲染的问题。