A lot of time the other person's mood will affect how well romantic interactions go over.
When you first meet up with his desired girlfriend, ask about her day. If it says she's having a bad day, it's a clear indicator that some romantic interactions won't go over well. You need to pay attention to the headline effects (the conversation/thought bubbles above her head) to find out what is bothering her. Your best bet is to invite her over and have a cooked meal waiting when she gets there. Call her to the meal by clicking on it and selecting "Call guests to meal" so she's well fed. After she's done, go watch television and invite her to join you. Make sure you let her use the bathroom if she needs to after the meal though.
Another good way to spur things along is to use the friendly interaction to ask her to spend the night. She'll use the fridge as she needs to, shower, and will even sleep (make sure to pick her day off for a sleepover). Spend the evening filling her fun bar, then get up early and cook breakfast and make your move after she eats.
If she's having a bad day because someone close to her has died, there isn't a lot you can do, outside of giving her a quick jolt with the moodlet manager.
