It's normal and this is where the manual helps.
After the troll lies down you need fire or acid to kill it. If you are at dragon eye and haven't found a fire sword till now then your best bet are spells (melf's acid arrow, burning hands, fireball, flame arrow and others...)
There are actually other ways... but those are bugs actually. The only proper way to kill it is fire, acid (or chunk it to pieces before it lies down, though at this stage of game you probably don't have fighters strong enough to do this). All other ways are bugs (in the sense they were not intended by the designers.)
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