Male not able to penetrate female

Help, can anyone tell me what else I can do?  I have a male maltese about 7 lbs mating with a malteshuz about the same size. My male has breed 3 other times successfully.  This is the females first time but she is ready and standing for him. He is thrusting and continues to mount her but cant seem to penetrate. I've tried to help guide him and he seems to be ok with it but still can't do the job. I've also tried to lift the female and put some lubrication on her,  but still no luck.... PLEASE can anyone tell me what else to do?
Desperate! 

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