One problem for us from the UK is that we don’t get our P60 (confirmation of earnings less tax paid) until April. How are we expected to submit our details before the end of March.
I’ve done it four times so far and never had too much of a problem. The first time was probably the worse because I had to show I could support her financially. On subsequent applications I’ve just included a letter confirming I would be supporting her during her stay and that my house would be her base, so no hotel bills to pay. The one thing that probably helped more than anything was the fact she had a good job and also a letter from her employer confirming he employment position and consent for her to take leave. We never used an agent, just double checked on all paperwork prior to submitting to UK immigration. If your gf hasn’t a job you may find it more difficult, good luck.