When you pot them you may be pressing them in too hard when adding soil thus crushing the stem base and roots. This is one of the reasons why that beautiful plant plug or otherwise purchased from the garden center perishes in a week or two when repotted or planted.

Also its been so very cold lately they may have rotted off

There are lots of problems that can happen to tiny plugs, the bigger ones are easier I think, more chance of survival

Hi Tintin, What plants are they, what size pots did you use, what medium was used and where are you keeping them ?
I buy plugs on a large scale and have always had a a near 99% success rate.

Hi, mini plugs are force grown, and have just left a very warm environment, and have only just passed the seedling stage, they need to be put in a warm greenhouse to start with, and the heat gradually reduced over several weeks until they can survive without any heat at all, they may need to be potted on into larger pots before being hardened off in a cold frame, prior to planting out in the garden at the end of may/ early june, Derek.

Without knowing what it is you do with your plug plants, no-one can really know how to properly advise you. We need to know what the plants are, what soil medium you're potting them into, where you're keeping them, and so on.

A common mistake is to plant them too deep, too. Only a few species appreciate that.

Thankyou for the helpful answers, plugs I was refering to were Salvias, I do pack them a bit hard, I also confess to putting in my greenhouse (bubble wrap lined). There was no trace of my heater, after 3/4 days and an equal amount of frosts, my wife reminded me that I had put it in the attic to store it. Heater now in place. plants looking a bit healthier on a night temp of 10c.

A big thankyou all for the answers to my problem, I have learned a lot from them, I seem to do all the wrong things when it comes to small plugs. Maybe I should wit till they are bigger before purchase.