Pond Skimmers

Can anyone help with the above. I have had 2 from different manufacturers both were the floating type and both had the the same problem namely 'rising up' sometimes after a very short while and when there is very little debris collected in the basket. I was advised buy the seller of the second one to 'weigh it down' which I have done but the problem persists. The skimmer is required to remove duck weed and other floating debris. Any advice, including a make known to work would be much appreciated