Configured the AWS API Extractor ETL, not able to see performance data or can only see a limited set of metrics

1. No performance metrics for the VMs

In order to check that information, log into your AWS console (using same user in the ETL) and check on the problematic VM if data is collected.
If not, review your AWS configuration

2. Only a subset of metrics collected for VMs (no memory metrics)

Referring to the official documentation
Entities, lookup information, and metrics for AWS API ETL  
by default, only these metrics are collected:

• CPU_UTIL

• NET_IN_BYTE_RATE

• NET_OUT_BYTE_RATE

• NET_BYTE_RATE

• DISK_READ_RATE

• DISK_WRITE_RATE

• DISK_TRANSFER_RATE

• DISK_IO_READ_RATE

• DISK_IO_WRITE_RATE

• DISK_IO_RATE

• GM_ON_NUM (Only for Autoscale)

If other metrics are needed, including memory metrics, the execution of the scripts for Amazon EC2 instances is required. This update will allow the following other metrics to be collected:

• MEM_REAL_UTIL

• MEM_REAL_USED

• SWAP_SPACE_UTIL

• SWAP_SPACE_USED     

• BYFS_USED_SPACE_PCT     

• BYFS_USED

• BYFS_FREE

• BYFS_SIZE

• TOTAL_FS_USED  

• TOTAL_FS_FREE      

• TOTAL_FS_SIZE

• TOTAL_FS_UTIL

3. Only few metrics are collected for VMs (possible: no memory metrics)

(a) Make sure cloudwatch agents are installed on the machines.
(b) Refer to official documentation: it is possible map and collect addional metrics if needed.