Looks like MultiStore stucks

Hi, I've met strange problem. Maybe it's related to data. BUt I'm not sure. I'm working with derivative in avro format so all "bad data" should be caught on early stages.
My pig script worked to 2 days each hour (invoked using oozie coordinator).
Now it stucks. It always have one reducer which shows progess = 67.55%
I see in TT log that it does merge, sort, then starts reduce.

I do use custom UDF in my pig script.
I've added counters trying to debug the situation.My UDF works with bags.
Counter says that UDF worked fine because "Reduce input groups" = "invocation times of UDF".

I even see counters of output:

Map-Reduce Framework
Combine input records	0
Combine output records	0
Reduce input groups	31 019
Reduce shuffle bytes	65 071 957
Reduce input records	96 727
Reduce output records	0
Spilled Records	0
CPU time spent (ms)	48 870
Physical memory (bytes) snapshot	643 358 720
Virtual memory (bytes) snapshot	3 821 920 256
Total committed heap usage (bytes)	1 057 357 824

MarkEndPointsForCurrentHour
callTimes	31 019
totalExecutionTime	13 690

MultiStoreCounters
Output records in _0_08	26 862
Output records in _1_09	7 383

Counters say that all data passed through my UDF and even some output has been written.
But reducer (always only 1 of 54 total reducers) stucks for 1 hour an then killed by JT because of timeout. All other 53 reducers finished in 7 minutes.

How can I debug MultiStore?