Does she feel like her partner loves her?
It took DP 15 months to tell me she loved me. I was her first real relationship after she'd gotten out of a really shitty engagement to a man to turned out to be rather unpleasant. For whatever reason, that left her rather wary of saying anything that might result in her being tied in.
I told her I loved her about 3 months in, and she jumped out of her skin so much there was very nearly a hole in the ceiling. I didn't get equivalent back from her for another year.
It didn't bother me, because while she couldn't say she loved me, I knew she felt it. Hell, the whole reason I said it was because unasked, she'd just gone completely our of her way to do something that would be an utter pain in the arse for her just to make my life a little bit easier. And there were lots of things like that. Most days something tiny, like a cup of tea in bed, sometimes something bigger. She turned up on Halloween in the half hour between finishing work and going out with her mates, just to bung some decorations up, give me a kiss, and piss off again. Not because I particularly needed Halloween decorations, but because she wanted to share her favourite daft holiday with me in some way.
So when on New Years Eve, having finally got newborn DD to sleep (Don't ask, we did everything in completely the wrong order!), lying in bed, she sleepily said "I love you", I just replied with "I know". Not to be some kind of smartarse, but just because I did know, and it hadn't even really registered that this was the first time I was hearing those words from her.
Any idiot can say the words, what really matters is whether you feel loved.