this is the train and development cell for multi-label classification task using Roberta (BERT). the first part is training and second part is development (validation). train_dataloader is my train dataset and dev_dataloader is development dataset. my question is: why train loss is decreasing step by step, but accuracy doesn’t increase so much? practically, accuracy is increasing until iterate 4, but train loss is decreasing until the last epoch (iterate). is this ok or there should be a problem?
train_loss_set = []
iterate = 4
for _ in trange(iterate, desc="Iterate"):
  model.train()

  train_loss = 0 
  nu_train_examples, nu_train_steps = 0, 0
  
  for step, batch in enumerate(train_dataloader):
    batch = tuple(t.to(device) for t in batch)
    batch_input_ids, batch_input_mask, batch_labels = batch
    optimizer.zero_grad()
    output = model(batch_input_ids, attention_mask=batch_input_mask)
    logits = output[0]
    loss_function = BCEWithLogitsLoss() 
    loss = loss_function(logits.view(-1,num_labels),batch_labels.type_as(logits).view(-1,num_labels))
    train_loss_set.append(loss.item())    
    loss.backward()
    optimizer.step()
    train_loss += loss.item()
    nu_train_examples += batch_input_ids.size(0)
    nu_train_steps += 1

  print("Train loss: {}".format(train_loss/nu_train_steps))

###############################################################################

  model.eval()
  logits_pred,true_labels,pred_labels,tokenized_texts = [],[],[],[]

  # Predict
  for i, batch in enumerate(dev_dataloader):
    batch = tuple(t.to(device) for t in batch)
    batch_input_ids, batch_input_mask, batch_labels = batch
    with torch.no_grad():
      out = model(batch_input_ids, attention_mask=batch_input_mask)
      batch_logit_pred = out[0]
      pred_label = torch.sigmoid(batch_logit_pred)
      batch_logit_pred = batch_logit_pred.detach().cpu().numpy()
      pred_label = pred_label.to('cpu').numpy()
      batch_labels = batch_labels.to('cpu').numpy()

    tokenized_texts.append(batch_input_ids)
    logits_pred.append(batch_logit_pred)
    true_labels.append(batch_labels)
    pred_labels.append(pred_label)

  pred_labels = [item for sublist in pred_labels for item in sublist]
  true_labels = [item for sublist in true_labels for item in sublist]
  threshold = 0.4
  pred_bools = [pl>threshold for pl in pred_labels]
  true_bools = [tl==1 for tl in true_labels]
  
  print("Accuracy is: ", jaccard_score(true_bools,pred_bools,average='samples'))
torch.save(model.state_dict(), 'bert_model')

and the outputs:
Iterate:   0%|          | 0/10 [00:00<?, ?it/s]

Train loss: 0.4024542534684801

/usr/local/lib/python3.6/dist-packages/sklearn/metrics/_classification.py:1272: UndefinedMetricWarning: Jaccard is ill-defined and being set to 0.0 in samples with no true or predicted labels. Use `zero_division` parameter to control this behavior.
  _warn_prf(average, modifier, msg_start, len(result))

Accuracy is:  0.5806403013182674

Iterate:  10%|█         | 1/10 [03:21<30:14, 201.64s/it]

Train loss: 0.2972540049911379
Accuracy is:  0.6091337099811676

Iterate:  20%|██        | 2/10 [06:49<27:07, 203.49s/it]

Train loss: 0.26178574864264137
Accuracy is:  0.608361581920904

Iterate:  30%|███       | 3/10 [10:17<23:53, 204.78s/it]

Train loss: 0.23612180122962365
Accuracy is:  0.6096717783158462

Iterate:  40%|████      | 4/10 [13:44<20:33, 205.66s/it]

Train loss: 0.21416303515434265
Accuracy is:  0.6046892655367231

Iterate:  50%|█████     | 5/10 [17:12<17:11, 206.27s/it]

Train loss: 0.1929110718982203
Accuracy is:  0.6030885122410546

Iterate:  60%|██████    | 6/10 [20:40<13:46, 206.74s/it]

Train loss: 0.17280191068465894
Accuracy is:  0.6003766478342749

Iterate:  70%|███████   | 7/10 [24:08<10:21, 207.04s/it]

Train loss: 0.1517329115446631
Accuracy is:  0.5864783427495291

Iterate:  80%|████████  | 8/10 [27:35<06:54, 207.23s/it]

Train loss: 0.12957811209705325
Accuracy is:  0.5818832391713747

Iterate:  90%|█████████ | 9/10 [31:03<03:27, 207.39s/it]

Train loss: 0.11256680189521162
Accuracy is:  0.5796045197740114

Iterate: 100%|██████████| 10/10 [34:31<00:00, 207.14s/it]