Upgrade from 1.7.4 to 2.3.2 fails, primary shards are not getting restored

when I restarted cluster, some of the primaries are properly restored, but some recovery task tasks hang

they are in the 'TRANSLOG' stage and are staying there for more than an hour, which is not expected since I run _flush before restart meaning there should be no remaining translog.

hot threads api shows below

0.0% (97.7micros out of 500ms) cpu usage by thread 'elasticsearch[IP][transport_client_timer][T#1]{Hashed wheel timer #1}'
 10/10 snapshots sharing following 5 elements
   java.lang.Thread.sleep(Native Method)

while /_cat/recovery shows below

INDEX_NAME 9 1688129 store translog n/a n/a 0 100.0% 0 100.0% 111 5087612156 0 -1.0% -1