Excess condensation that causes the lid to 'float'

https://www.youtube.com/watch?v=lsY0FbT8zls
To close the lid, place it on the pot making sure to align the arrow on the lid with the arrow on the lower handle. Make sure the lid is evenly placed on the top rim of the pot. Turn the lid clockwise until the handle on the lid and the lower handle on the pot meet, the lock will automatically click into place.
https://www.youtube.com/watch?v=F_4nzDomIP0

The Floating mini gasket is a silicon ring that is placed on the floating valve, a metal piece located in the lid is it missing or damaged in some way preventing the lock

You should have a fit that has a little wiggle room, including a little space around the edge of the lid. When you cook in a slow cooker, you'll make a bit of steam, and it needs room to escape. A super tight lid would likely end up 'burping', and possibly spraying spurts of whatever your sauce is, across your kitchen. Having that small groove there also leaves you a space where a bit of steam can condense and either trickle back into the pot to keep things moist, or just sit there in the gap and not drip all over your counter. Having a little bit of give also means that if you cook and let things cool, the condensing steam won't create a partial vacuum and leave you having to plant a foot on the side of the crock pot to pull the lid off (likely with more spectacular spew-age ready for viral videos).

Cheers!

Most crock pots are made of porous ceramic and is relatively brittle. It is glazed on all sides (except the bottom usually), to make it hold liquids without absorption. The lids, usually glass or thermoplastic, sit of top, but do not usually fit tightly as this could crack the crock. Also, it is not a pressure cooker, so you want excess steam to vent. The lid just keeps the heat inside and allows condensation to drip back into the crock, reducing the speed at which the fluids cook off, thus the term slow cooker.

No liquid added? That me be the problem.

Your gasket is no good and needs to be repaired. The water is supposed to be pressurized as high heat steam and when pressure is excessive the valve does its thing to lower to acceptable levels.

No it does not need to be tight, excess steam needs to escape.

Rival Glass Crock-Pot Lid Oval
Fits Rival Crock-Pots using a solid oval glass lid cover
Lid Measures approximately 12-1/4" x 9-5/8" from end to end
Lid is glass with a metal rim
If this sounds like it click on the web link below
http://buyitnow64.stores.yahoo.net/ricrpotslcog2.html
OR IT COULD BE THIS ONE
Rival Crock Pot Slow Cooker Replacement Round Glass Lid Cover
Works with Rival Crock Pot Models:
3300, 3302, 3305, 3351, 3350, 3355, 3504, 3520, 3550, 3500, 3654, 3656, 3760, 3845, 3850, 3950
Approximate total diameter: 10"
Use model number NOT measurements for the correct replacement part
PLEASE NOTE: If your model number has a slash or a hyphen and another number on the end of the numbers above (ex. 3300/2), then this lid will NOT work.
If it sounds like this one click on the weblink below
http://buyitnow64.stores.yahoo.net/ricrpotlidre2.html
Hope this may help.

Hello,
I would suggust that you take your crock pot to your nearest service center. To see if they can correct the problem with your crock pot.
Okay. Spurs21

You should keep the lid on as much as possible. You are correct; once you open the lid, the liquid inside can quickly lose temperature at a rate of 10-15 degrees per minute.

Additionally, taking the top off allows steam to escape, which reduces the point of cooking in a slow cooker (cooking in a relatively closed system, with the steam condensing back to water and flowing back into the broth).

It's not doing a whole lot to the flavor (aside from causing the stew to thicken with the loss of water), but it is definitely causing the cooking to take longer than it should.