What is going on with my fiancée in regards to my children?

I have been engaged to my fiancée for 1½ years now. We were engaged after 4 months together…we have known each other since we were very young though. Both of us have been married and divorced once in the past. Only difference is I ended up with 3 children from my marriage.
In the past there weren’t issues with him having my children over to our house every other weekend (as agreed to in my parenting agreement) as of the past 4 months or so though he hasn’t, seemingly, wanted them around for one reason or another (ie: too tired from work, not feeling well, etc.) so I haven’t seen them for the last 4 months. I have talked with him about this several times. I get the same response. He likes my kids, thinks they are great and fun and he especially likes them since they are ½ me. But he refers to them as my x’s “puppies” at other times.
We are supposed to be married this coming June. He and I want a child of our own, this scares me though because I don’t want my other children to be put to the back burner. If anything I want them to be more of a part of my life. I love my fiancée. He cares for me emotionally, physically and financially better than I have ever been cared for. When my children are with us he is wonderful with them and they love to be with him, too.
I’m just looking for ideas as to what his hang ups with my kids being around might be and if there’s another way I should approach him with this issue to get him to understand me better. Apparently the tears aren’t working. Thanks in advance!