Depending on the variety, it can take 7 years or more for production to begin.

A lot of things can affect pollination, including weather at the time the flowers are budding or in bloom and with the availability of any pollinating insects.

I can't answer your second question fully but cross pollination by another species rarely would account for lack of fruit production. In fact, with the pears it is quite the opposite....cross pollination of the ornamentals by the fruiting species accounts for fruit and seed production on the ornamentals, specifically the Pyrus calleryana cultivars, and contributes significantly to their invasive properties.

You can graft cuttings, called scions, to your ornamental pear tree. And since those trees, presumably are older or mature trees you'll start getting fruit much quicker. And it's a way to back up your pear selections. Late winter is a good time to take cuttings which is when you might be trimming the trees anyway. Store the cuttings in a ziplock bag with a damp paper towel.