I have a Q&A website, and facing an issue google search engine indexing, please have a look at following use case:
1. User Posted a question: Question gets submitted to Google for indexing through xml sitemap
2. Suppose next day somebody answered that question
Now what happening if my question gets indexed before answering the google is not updating its cache as soon as some change happen in the content, please let me know how can I tell google to recrawl content of my page as soon as somebody add new answer to it?
Your site is a Q&A so I presume that every time someone create a new Question it will generate a unique URL for that.
Indepentend if you have or not an answer you want Google to crawl and index your site as quick as possible to start to analyzing the new page and bring traffic to it.
The best way to expedite this "re-crawling" is to use a ping service that you can trigger after your user answer the question.
PS: If you site is updated frequently you shouldn't have a problem with crawling, because Google usually identify this type of website really quick.
Drop me a call is free for this week.
You can Re-Submit sitemap through Webmaster or use Fetch as Google Bot option in GWT.
Your concern is very common and not knowing the site and not knowing its structure, age or underlying technology I hope below will help.
You can't force google really to crawl your site. Nor can you force it to crawl your page.
You can resubmit your sitemap and hope this it will get crawled. The best method is to be active on your website in general and Google will crawl it with more regularly.
If your site is light on content and pretty new (weeks) then it won't get crawled as often as you would like.
You will read all sorts of stuff about how you could fool Google but from my experience the thing to is to keep a regular content stream going and that will indicate to Google that you are an active site.
You may ask what about the specific questions and the answers then don't worry Google will get to it if you are creating content in other areas.
Let me give you a recent example of one of my niche sites.
I created the site initially with 10-12 core pages. - Day 1 - Week 1
I then over the following days created 2-3 posts / pages on the site - Day 2-Day14
The site pages weren't visible till about Day 4-5.
I continue on a daily basis to create the best content possible. Length helps of course but don't create words just for the sake of it.
You may sya that your Q&A website is a different model but I would look at the big Q&A websites like Quora and look at some of their core pages that surround the site and creat them.
If you are light on questions then reach out to your network and ask them to either ask questions or help with answers.
Glad to do a quick call with you if this is unclear.