Dumb Question - 250 Watt MH - Replacing the bulb

[phar]

So a friend of mine gave me a Aquamedic 250 Watt MH fixture. I thought I'd upgrade from my 150 Watt. I bought some new bulbs for it but they don't seem to snap in. They push down into place but kind of rest there. I know as soon as a I flip the fixture they are going to fall out. They almost feel like they are hitting something in the plug.
I've pushed down with moderate pressure but nothing seems to snap into place. I've pushed down as hard as I dare without breaking them.
What am I missing here?