Why won't my car shift into second gear?

I have a 2012 VW Passat SE with the 2.5l 5cylinder, automatic transmission and 48k
miles. I bought this used with 45k miles and within the last 3 months we've
experienced an issue where one of us(my wife or I) will accelerate onto a road and the
rpm's get to about 4000 but the car wont shift into second gear. I've tried letting off the
gas, hammering on it, etc..... Sometimes shifting in to S will allow me to manually
bump it up into 2nd, 3rd, and so on but recently that stopped working. There are no
warning lights or other indicator lights lit on the dash. The only common thing we've
been able to piece together is that it's more likely to happen if it has rained recently.
We are going to take it back to the dealer that we bought it from but are looking for
any help or advice.