I'm by far no expert, however, there are a couple of things you should check.
make sure you have the correct altcoin-qt client for whichever coin you are mining with.
Also, make sure the altcoin in question has not branched off into another block-chain and thus you may have been syncing to the *incorrect* block-chain.
make sure you have the correct super-nodes IP addresses inside your *.conf file from whom you are syncing with.
EXAMPLE *.CONF
Code:
rpcuser=doge <-- check this is correct for your altcoin
rpcpassword=wow <-- check this is correct for your altcoin
rpcport=22555 <-- check this is correct for your altcoin
rpcallowip=127.0.0.1 <-- Maybe need to add this rule
rpcallowip=192.168.*.* <-- Maybe need to add this rule
addnode=95.85.29.144 <-- Super-node for this specific altcoin (DOGE)
addnode=162.243.113.110
addnode=146.185.181.114
addnode=188.165.19.28
addnode=166.78.155.36
addnode=doge.netcodepool.org
addnode=doge.cryptoculture.net
addnode=dogepool.pw
addnode=78.46.57.132
daemon=1
also check you have allowed inbound traffic access to these IP-nodes (or at the very least, these nodes are *not* being blocked).
check the RPC ports are configured correctly..
If you still have no luck check this forum for help or try readit threads for your specific altcoin which you are having issues syncing with.
If all else fails try to see if there is a bootstrap.dat of the block chain which you can download yourself and sync with your client manually (faster method)