Online Price Difference

Trying to book UC tickets online & even using several different dates, when we get to the payment option it’s saying fare change & the price is no longer available. This has been happening over a couple of days & when we call VS they say the price has increase (significantly) & that there’s nothing they can do to honour the online price. Is this a common occurrence? According to Expert Flyer, some of the dates we’re looking at don’t even have a single seat sold so don’t believe it’s because the allocation has gone.