I suspect finding past threads about visas that were refused, and what if anything was done to address the refusal, may help you to understand what you need to provide.
I've just dug out the refusal letter from our first visitor visa application made in March 2013 and thought I'd share it with you so you don't make the same mistakes. Reasons given for refusal were:
1. She was unemployed and totally dependent on the money I sent to support her, but I failed to provide anything to show regular money transfers.
2. They were happy from the photos provided that we'd actually met, but I failed to provide evidence of the claimed daily communications in the form of chat logs from Facebook, Skype, Line.
3. I provided 'WorldSIM' phone bills showing regular calls, but the number didn't match the number I declared as my mobile number.
4. We'd mentioned visiting her aunt in the UK but gave no evidence of the aunt's immigration status or of her having knowledge of our visit.
In May 2013 we re-applied, going to great lengths to address all the points raised in the previous refusal. My letter of invitation referred back the the refusal and explained what steps we'd taken to address the concerns raised. These included:
1. Bank statements showing a fund transfer to Thailand and ATM withdrawals made whilst I was on holiday in Thailand, and Western Union transfer receipts. We also included her Bank statements showing receipt of these funds.
2. Screenshots of chat logs on Facebook (21 pages) with appropriate content showing dates to prove regular contact during 2 sample periods in Dec 2012 and Apr 2013, and an edited Skype log (11 pages down from 160 removing 99% of the text chat) showing call start, end and duration of video calls proving daily communication.
3. I explained that I used WorldSIM for cheap international calls, but that I retained my 'Orange' SIM for UK calls. I copied the SIM wallet to show the phone number, and screenshot the registration page showing it in my name.
4. I explained we wouldn't be staying with her aunt, merely visiting, and included an email from her welcoming a visit. I also copied her British passport.
While OldGit has pointed out that it is your girlfriends application not yours, of the supporting documentation we provided, 90% came from me with her providing only ID (passport, ID card and house book) and a bank statement. I, on the other hand, was listed as the source of evidence proving a genuine relationship (photos, phone bills, chat logs), financial support (bank statements, money transfer receipts), proof of finances to fund the trip (bank statements, wage slips, P60, mortgage statement with evidence of overpayment), the sponsors letter of invitation and a few other bits (quotes for return flights, quotes for travel insurance which in the end we didn't buy so a trip to the Doctor cost us in excess of £80 for a sore throat diagnosis).
Hope this is of some help to you and others trying to sort their first visit visas. I appreciate it's an old thread.