Me too, me too!! You're not alone DL, though the responses so far would indicate it's a rare problem. SOrry, have to go incognito on my reply cos I find it quite embarrassing to admit difficulties in kissing!
DH and I used to kiss quite a lot, like all couples at the start of a relationship, but the amount of real snogging slowly diminished. I think that's fairly normal too, but I know what you're referring to is the actual enjoyment of it, not the amount. I sound like a female you. I LOVE kissing - but DH is not as into it as me. He too has thin lips and also rather a small mouth so full-on snogs don't come naturally; they require effort on my part to get his mouth to open enough to accommodate mine, euugh sorry that sounds gross!!
I love using tongues yet I think he'd be happy just with 'dry kisses', but there's NO WAY I'm having the rest of my life with no proper snogging. Because of this incompatibilty I often feel very conscious of how we're kssing when we are, hence it's not so enjoyable. Like you, I had no problem with previous partners. I remember once snogging for 40 mins non-stop with an old boyfriend and it was GREAT!
Last night we were having a cuddle on the sofa and I started kissing him and I told DH to open his mouth wider. He did, and it was really nice.
Solutions? Well, talking about it has to be the start. If you can't do that then any solutions are impossible. Personally, I find that pretending your partner is someone you don't know well, like a first snog with a boyfriend, makes the passion rise more and hence more tongue action . Find out if she's always disliked tongues or is it just with you she doesn't like it? If it's the latter then maybe reconsider your technique. If you can get her to talk about it you could maybe suggest having some 'practice sessions' of kissing, trying out different ways until you find happy medium like a less intrusive tongue yet still some, er, moisture!
If you can't talk about it and have her understand how important it is to you then I reckon it just ain't gonna improve.