Yep, it is as Terier correctly advised. Easy answer is a 50 amp plug and a 30 amp plug are different.
You can buy adaptors that will let you plug a 30 amp into a 50 amp, but you probably only need to do that if something is wrong with the 30 amp receptacle.
Another habit to form, is to make sure the breaker is turned off for your plug, while you connect your trailer cord to it....then turn the breaker back on. (See pic)
1. Breaker would control the 50 amp
2. Should control the 30 amp
3. Should control the 20 amp