Please end your thoughts with periods so the rest of us can read them with greater ease. That one time out of every five is a time I would guess she is not wet enough, and the two of you have not gotten her sufficiently aroused and ready. I would venture to guess that she is fairly inexperienced, particularly with a lover of your dimensions, and does not know what she needs in order to resolve the pinching issue, nor does she know that she is insufficiently wet and aroused for penetration. You do not mention her level of experience.
One thing your experience may have failed to teach you is that nothing works for everyone. You can never really know what you're doing. Each new partner has different likes, dislikes, needs, responses, and desires. She needs something other than oral sex to get her going before penetration; she is too inhibited by receiving oral at this time, though I encourage you to keep encouraging her to be more open to the experience if she's really found it pleasurable with you in the past. Meanwhile, the two of you need to figure out what her other triggers are. It seems to me that she needs to be far more excited before you begin than she is at the moment, that one time out of five.