NaOH CO2 trap troubleshooting

I was wondering if anyone could help troubleshoot an experiment I'm working on.


I'm running an experiment to measure the microbial respiration of compost samples using a $\ce{NaOH}$ trap. Each sample is in a jar with 20 ml of 1M $\ce{NaOH}$ and is placed in an incubator for 24 hrs. After 24 hrs, 20ml of .5N BaCl2 and an indicator are added to the $\ce{NaOH}$ and it is then titrated with $\ce{HCl}$. I run a few blanks each time, with an $\ce{NaOH}$ trap in a jar without a compost sample. This is repeated over 4 days In theory, I should need less $\ce{HCl}$ to neutralize the $\ce{NaOH}$ which has been sitting with the compost samples then the $\ce{NaOH}$ just by itself. Sadly, after hours and hours of work in the lab, this has not been the case!


I have 45 different samples of different types of compost. Each time I run the test, about a third of the samples require more $\ce{HCl}$ to neutralize than the blank.


The blanks are all consistent in the amount of $\ce{HCl}$ needed, I ran three last night and one the night before. The test also seems to be picking up samples which are respiring heavily- when there are moldy samples, they require only a small amount of $\ce{HCl}$. I can't really decipher a trend with the ones that require more $\ce{HCl}$ to titrate than the blank other than that it's somewhat consistent. 41% of the samples which needed more $\ce{HCl}$ than the blank on day 2 also needed more on day 1, and of the day 2 "error samples", 88% were from a treatment which had an "error sample" on day 1.


I thought perhaps this had to do with ammonia volatilizing from some of the samples, but this doesn't necessarily make sense with the samples which are taking more $\ce{HCl}$ than the blank. I also noticed that on the samples which require more $\ce{HCl}$ to titrate than the blank, there seems to be more precipitate (barium carbonate?) that settles out after the titration.


Do you have any idea what could be happening or what I am doing incorrectly? It's definitely a long process to do this measurement for 48 replicates over 4 days - I'm hoping the data isn't totally worthless.


Thanks


Edit: Pictures


[![enter image description here](https://i.stack.imgur.com/QqLtH.jpg)](https://i.stack.imgur.com/QqLtH.jpg)


On the left are samples which required less HCl to neutralize than the blank. (I forgot to add an extra 20ml of BaCl2 to the second from left, which I had added an additional 10 ml NaOH to, so that one is an error). In the middle is the blank and a sample which required the same amount of HCl to neutralize as the blank. Then on the right are samples which required more HCl than the blank to neutralize.


