Why did Maria Antoinette's husband have a hard time giving her a baby?

Because Louis XVI had a certain problem. The foreskin of his penis was too tight, so erousel and actual intercourse was impossible and very painfull. For years his doctors and Marie Antoinette almost begged him to have it fixed by a small operation (circumcision) but he didn't want to because he was afraid of the operation. Finally, after 7 years, the circumcision was done and the marriage was consumated (Marie Antoinette's brother Leopold had a very firm talk to Louis XVI once he was visiting Versailles and that was exactly what Louis XVI needed to face his fears and have the operation take place).

1 year later, Marie Antoinette would be pregnant of their first child.