My dryer squeaks and slips before it starts to spin-is it the belt?

The belt in a clothes dryer is tightened by spring tension. There is nothing for you to tighten. Clothes dryers do have bearings on the drum and it may be that yours are in difficulty.

Sounds like the belt or that the repair guy did something wrong. Call them back!

You may need to replace the tensioner spring, or it may not be hooked in place like it should be.

sounds like tensioner pulley

This is happening to my dryer as well! Would love to know why

Type in your model number and make and follow it by trouble shooting squeaking noise. There might be forums or even You Tube videos that can diagnose your problem. Some videos even show how to fix the problem and have a link for a part to buy, if needed. We have done this for a Bosch dishwasher and a Fisher Paykell washer and dryer. Good luck.

It's possible the tensioner arm or tensioner wheel are sticking and not creating enough tension. Lubricating the arm and wheel with WD40 should loosen them. Also when the belt was replaced the tensioner arm may have accidentally been bent and is binding. If yours does not have a tensioner arm and the belt is direct from motor to drum. The motor needs to be adjusted to tighten the belt. Loosen the bolts holding the motor and slide the motor away from the drum, hold it in place, and tighten the bolts. There should be about 1/2" deflection when pressing on the belt with you fingers.

Note: WD40 is NOT a lubricant - if you need to lubricate something, use the recommended oil or grease product. If the proper lubricant is not documented in your paperwork, just call the mfg and they will tell you what to use. https://lifehacker.com/5891936/when-should-i-not-use-wd-40

WD40 is also flamable. A dryer is definitly not a place to use it.